Sorry to read this Jez, had to do the same to pay the stamp duty when we bought a couple of years back.
Sorry to be the voice of reason, but a collections value and what you can quickly liquidate it for are two different things. You will not sell 80 carded figures at one event unless prices are stupidly cheap. You have some great items but also a lot that are hard to sell for good money (eg the Kenner 77 backs) so if you're looking for a quick sale cash value, you need to prepare yourself for well under market value - perhaps 40-50% which is a common "trade price."
Two bits of advice - don't sell anything you can't replace, and sell the expensive but easy to replace (eg 12 back luke) first. It's the £1000 MOCs that will dent a deposit, not the £25 ones! Then if you still need to sell the rest in bulk to a dealer.
Good luck!