There are two companies I'd use in London, but neither will be cheap.
http://www.johnjones.co.uk are excellent and very well regarded, although massively busy and rather pricey. You'll need an appointment to go down and see them, but it will be worth it. JJ are pretty much the go to name for non-antique framing where budget is secondary.
http://pauliframes.com are also very good and well regarded (I have bought items framed by Pauli without worrying, normally you don't buy framed as it can hid a multitude of problems).
I also use mrframeman.co.uk regularly who collects from London a couple of times a week but is based in Norwich. Gary is cheaper than the others and pretty well regarded, but again hugely busy so you could wait a good 6-8 weeks to get your pieces back. He'll email options over once he has the art in hand, but it helps if you have an idea of what you want first.
I've had tons of stuff framed (I collect art) and would advise the following from whoever you use:
1. Get your item conservation framed to museum standard. Always ask for conservation framing and make sure the receipt says this - some auction houses insist on it to sell as there is some comeback on the framer.
2. Make sure it is completely, 100%, reversible (conservation should be)
3. Get them to use hinges and not tape.
4. Make sure they understand the value and have insurance while it's in their possession (the guys above all deal with Banksy, Os Gemeos, Kaws etc, so know the drill - but it doesn't hurt to remind them, especially as they won't have seen loads of SW).
5. Use 'museum glass' if your budget stretches. 'Truvue Museum' is a nicest at a semi budget level (IMO) - its reflection free glass and effectively looks invisible. It will break if dropped or banged hard (like any glass) so for massive pieces or if you have toddlers, you can get a museum acrylic - glass is nicer looking though.
6. If museum glass is a bit pricey, UV glass is a decent second option.
7. You don't have to have a mount over the top of the piece, its easier for the framer so they might steer you towards this. Look at float mounting as an alternative - most of my art is float mounted.
8. You can make the frame much deeper looking by having spacers put in - again, I normally go this route.
9. Definitely always make sure the art NEVER touches the glass. A little moisture in the atmosphere and you're screwed. All art moves, expands, contracts etc - give it space to!
Good luck! I'd post some photos, but they'd be about as far off topic as something could get!
Bad framing can destroy paper/art - I've had tape used which tore the back of a fairly valuable print. I've also had the frame 'pinch' the art, which again damaged a piece. This was a while ago and I learned my lesson the hard way - I'm a framing nazi now.
Good framing, IMO, absolutely makes the piece - I had a commission done a while ago, I waited about 18 months for the guy to do his thing, paid and then was a bit 'meh' when it arrived. I was already in for a good bit, so thought sod it and got it framed (by mrframeman) - when I got it back I was blown away - the framing completely changed it for the better.
