I think the main focus of my questions would be based on costs, infrastructure and reliability.
Obviously, if I was a high frequency algo guy, I'd be looking for low latency and redundancy options. I have no experience in this type of trading though, so I'd be looking into the costs per trade/RT and desk fees.
Trading LSE equities, £1.20 - 1.60 per filled transaction + exchange fee for aggressor charges is reasonable for prop firm. I am not so sure about futures but, from what I've heard, around £0.50 per round turn is relatively good for a high volume scalper (most retail firms charge around £5 per RT). However, these prices are for if you are backed by the house - I'm not sure if the charges would come down if you risk your own cash.
Regarding a desk and trading setup - anything from £1500 - £3000 per month is about right depending on the use of TT/RTS or other trading software, Reuters/Bloomberg info terminals, CQG charting, Squawk, etc.