How would you invest in British theatre?

To answer the post "A new comedy with a Hollywood name as the lead" would by my safest bet for a profitable return. Comedy is probably the easiest sell across the board and a name from Hollywood would probably have an equal or better draw to "All-Star British" or stunt casting.
It does depend entirely on how you define "All-star" and "stunt casting". My interpretation of All Star would probably be Hollywood or legendary names, whereas others would (and do) use that term a great deal looser.

In a hypothetical reality - to answer the thread title, rather than the post - I'd actually spend my money on musical development and hope to hit a point of break-even.

I too would prefer to spread the money and perhaps to theatres whose work I wish to encourage and which appear to lack monied supporters, eg Southwark Playhouse. I know all theatres need more money (and keep asking for it) but some other venues I visit regularly seem to have big-name funding in place and/or more wealthy clientele, or give a less welcoming impression.

It's two separate questions really, isn't it? How I would invest £100k if I hoped to get my money back is completely different to how I would invest £100k in theatre in a way that I wanted to. (I would absolutely not invest any money in theatre at all if I hoped to get any money back; I don't even pay out for theatre memberships in the hopes of benefiting from perks, and those are much more guaranteed than the possibility of making money from ticket sales.) Also, as I think we've all recently learned, £100k really isn't very much at all when it comes to professionally producing theatre! Honestly, assuming I've got £100k to spare, I would probably just donate to theatre-makers I know personally who are heavily under-represented in the field. I'm not going to make any money back, but I can at least help alleviate the weight on the shoulders of those who are trying to tell stories that we haven't heard a million times before from the same posh crowd who seem to get all the funding as easily as they appear to get everything else in life.

I would love a new British musical with an incredible cast. Not necessarily any big names but if there are then the right people for the job, not stunt casting.

I would not put hard earned (and tax paid) money into theatre. Unless you find a story or show that you love and would see multiple times yourself. Burleybear knows great theatre when he sees it.

Cult show done on the cheap with loads and loads of merch.

I'm requesting it is salt and vinegar.

Having had a think about this, I would be tempted to invest my money in upgrading the theatre seating to include the ability to allow patrons to give electric shocks to those patrons who are badly behaved during performances. Ok, there is no financial benefit, but at least you get the emotional benefit of being able to watch the performance in peace.

I wonder if this is something Nottingham Playhouse is trying? It is a bit dodgy extrapolating a trend from two items but they had Mark Gattis in "Madness of King George III" and are about to have Alex Kingston in "An Enemy of the People“

Oh definitely. A short, sharp jolt of electricity to render them unconscious but not enough to fry them. The burn marks would ruin the seating upholstery and leave a smell of rotting flesh in the air.

No! I love 70s concrete. There must be plenty of places where they could install more bogs. The Donmar is terrible (no seat rake either) but is going to be restructured soon, according to a thread on this site.

Ok, ok, I will leave you a 3D mock up you can enjoy. Or you could maybe help me put up signs which state clearly where the entrances are, the exits, the directions of the corridors, move the toilets and triple the number with proper rest areas for patrons to take off their corsets if ness so with seats in the rest rooms and so on... provide proper restaurants, pref several all looking on to the beautiful river view, surely one of the most expensive views in Europe, not at present viewable from inside the building. Provide a proper theatre book shop, with all the audio and dvd and streaming tech of plays from the past, don’t argue, with ample seating inside and outside the building. Then may we have theatres with excellent acoustics, from any seat, perfect mechanics and ultra modern tech with future proofing. At present you cant hear from the side of the Lyttleton and the circle of the Olivier, hence all the miking going on there. Funny how I could hear every word crisp as can be in the Duke of York’s last night. Moveable seating but comfortable with leg room and aids for hearing problems built in etc. If you want a studio theatre, then have one and don’t pretend it is for arrogant directors who want a sold out sign on their shows cos so few seats for what are in fact major productions. Ditto playwrights. Make a car park, taxi, little bus facility to from stations, including the tube, and easy payment techie ie on your bl***y phone. Make the pedestrian ways to the different theatres differentiated and disabled friendly. Make the lifts work. More lifts. Put a garden on the top. Build it with total up the minute and future proofed energy supplies. Put a bee hive on the top. I could go on and I have mixed up the categories but you get my drift. Some of the problems are because they never allowed enough dosh for the place and some because it wasn’t built by theatre people despite one or two actors being involved. Of course Olivier wanted his theatre to be a huge amphitheatre - who wouldn’t ? But it was a serious mistake and self indulgent.
I haven’t got the money of course and looks like we never, as a society ever will have the money again to build a National Theatre. Such a shame our one chance was taken by people from the long gone ..and I say that with respect, I do. They were post war in chronology only, not in mind set.