I am no expert but my two pence..
20,000 pounds is indeed unthinkable & unacceptable. I can get that much if I continue to stay in India for 2 years (albeit with two job changes). The annual hike is anyways much better here compared to UK.
The point I am making is that one doesn't want to cast himself in a particular salary bracket whilst in UK. And if one thinks 'I will accept any offer that comes along at this time and ask for 40,000 pounds when the market recovers' would just be the individuals whim (Unless one is 'really' satisfied with whatever s/he gets!). Better start with a decent amount so that you can ask for a reasonable increase during the hike or applying for further job.
Next time I reach UK, I do not plan to take 'pictures' like tourists. I mean 'business'
I do not mind if the market takes pretty long to recover. I am sharpening my skills while I am still here !!