Just some unconnected ramblings for consideration.
£20,000 is not a huge budget and, in that price bracket, you will be lucky to find a boat that needs nothing spending on it.
You need to budget for Suveys and lift outs (as you have found) and each time reduces your budget by £500/£800
You will almost certainlyneed to budget for Blacking (£400)
You may well need to budget for the Licence £700+
You may well need to budget, in the near future, for the BSS
You will need to save (say) £2000 for unexpected repairs in the first few months.
This basically means that you should be looking at boats around £16k-£17k with the hope you can get them for £15k
The further down the price scale you go, the more it will cost you in the future.
EG -plating costs that were identified as £3-£4k actually turned out to be above £10k once the work started.
A 2008 boat at under £20k - there is something wrong with it.
This all comes across as negative (and I suppose it is) but it is actually intended to help you - I reckon for every £1000 you can increase your budget you will save much more than that in running/maintenance/breakdown costs.
We all know that "want it now" feeling, but try and save up for another 6 months, or 12 months - there are a lot of good boats out there at £25k
Spend every penny on a good (well known makers) hull and solid reliable engine. You can live with the insides (as long as its dry and warm) and do a bit of decorating etc as the money comes in.
Good hunting, and good luck.