Well, I can give some generalized advice about mortgage shopping. I was a mortgage counselor for a little over a year. Nothing UK specific I'm afraid since I'm in the US, but these tips should apply in the UK as well.
1) Credit Unions are usually a great choice. They tend to have lower rates and they generally have lower closing costs too.
2) Dunno if the UK uses a point system like the US does, but usually lender quote a rate that sounds really really good, but don't ention that the rate includes points. Points are composed of two parts: Origination fee and the other one which I have temporarily forgotten the name of. Ask them if the rate they are quoting includes points. Each 1 point is 1% of the loan amount, so it adds up.
3) Ask for a breakdown of the closing costs. This is where most lenders really stick it to you. They add in a bunch of costs that make no sense, or can be cheaper elsewhere or on your own.
4) Beware Adjustable Rate Mortgages (ARM's), make sure of what you're getting yourself into. Ask if there is a term in the loan contract that limits your refinancing options (some of the more unscroupolous lenders have a condition in the contract stating you can't refinance for a set number of years), ask if there are any prepayment penalties.
5) If buying a house, ask if the seller will cover a portion of the closing costs. (Mostly depends on whether it's a buyer's or seller's market)
6) Go with well-known institutions. If it sounds too good to be true, take a second, third and fourth look at it, and do some web research.
Any other questions, just lemme know and I'l help as best as I can