For my deed poll I got a friend who is a qualified teacher to witness it. I've only needed deed polls and my GP's letter to change my name, title and gender marker on every piece of id I have: passport, driving licence, professional certificates/log book, banks etc.
The only statutory declaration that I've need was the declaration of truth when completing my gender recognition certificate application, if you go to a solicitor the standard charge is £5. Obviously after I have this I will have a new birth certificate and everything legal will be in my current name and the deed polls will no longer be needed as I progress with all the relevant government id in place.
I recommend you go to one of the websites, the deed polls they provide are nicely written, you can ask for 10 + of them (i recommend 5-10, no more is required). Then just send them out with your name change letters and GP's letter; also not one place has kept the originals and they always send them back to you.