How to finance an actor showcase
I stage and promote these kind of shows on a regular basis, and in my experience an actor's showcase doesn't have to be funded at all. All that is needed to create an industry-standard performance is a group of like-minded individuals, scenes (new writing is a good idea), and a large-sized room in a pretty central location (near a tube if possible - it's essential that the venue is easily accessible). Many pubs have spaces that are suitable, or that can easily be adapted, and if it is early in the week (Mon-Weds), are happy to give it to you for free (they will be grateful for the extra trade from actors and audience). The venue itself probably won't add kudos to your endeavour, which means you will have to sell it to the industry - hard!
Of course, if needed, the participating actors/writers can simply chip in some money as a collective, say £25 each, and this revenue - if there are at least 10 participants - will fund the rental for a dark night at a theatre (or before - or after - someone else's show, normally on their set).
Many people in the industry sell showcases to actors - often expensively - and it is a viable way to raise funds. A venue with an excellent reputation can be hired, and a decent profit also made. The downside is that it can unrealistically heighten expectations, and unless the producers can guarantee a healthy attendance from industry professionals, will sometimes lead to disappointment and a poor reputation.
Two words of advice: if possible try to avoid staging or acting in showcases on a Saturday afternoon or on a Sunday (even though it is sometimes easier to hire spaces then). It is really quite rare for established casting directors and agents to attend shows of this kind on a weekend.
And finally, check which shows are opening around the dates you are thinking of staging your showcase: it is all too likely for gigs like these to be completely overlooked by the industry during a period of big openings and premieres.