Where can I listen to some cans? (UK)

I'd just go for a Hi-Fi meet. Most shops I've seen have a pathetic selection of headphones, only a few from the same range and never entire range of several makes.

You shouldn't have too much trouble finding shops that demo Senn. As for Grado, there are some shops that demo it but probably not the higher models (be aware that they might not have the new 'i' series and try to flock you the older stock). As for AKG, I seen them on sale but no idea where you will find place that demo it. Best bet is to call the Senn. or some local hifi stores to see if they stock/demo some of the phones. Note though, shops probably do not have any decent headphone amp so won't be of great help unfortunately....

OK, looks like you have to travel to London then... I think Walrus Systems in London, may have Grado, Stax and Graham Slee on show, Richard Sounds in Swiss Cottage might have Grado. As for Senn. I have seen some a shop in Camden has model up to HD650 (but no amp to do it justice though). The best thing to do is call some shops in London and make sure they have the headphone on show and also you need to know what headphone amp they got. Note normal integrated amp headphone jack will not be able to deal with phone like 701 or 650 etc. Also you could call some shops in Tottenham Court Road to see what they got in store - fair chance of finding AKG, Senn. & Grado. I know Arena Electronica is able to get you the goodies but not sure what they have in stock though. Re: Grado, shops most likely only have lower model on stock.

Don't walk in demo them, walk out and buy them online to save money.

Generally internet is better price wise but sometimes bricks n mortar shop is ok too - got a fair deal with my DX1000 from Arena and there is virtually no UK e-shops selling it...

I think its Shasonic on Tottenham Court Road that has a specialist headphone section. They have a lot of AKG, Audio Technica, Grado and some Sennheisers you could give them ago. Their prices are high but I'm pretty sure your can barter.

Digital village sell a lot of headphones but I'm not sure if they let you demo. I got the store to order me in K501's and K271's, you pay the same as their web store too which is pretty much the cheapest for the headphones they stock

Shasonic are excellent and have always been quick to come down from the sticker price in my experience. There's also a shop next door (I think its next door, may be one or two doors down) that sells the new "i" Grados for better than online prices if you haggle.

I seem to remember Shasonic having demo headphones available, not sure how you'd fare elsewhere on TCR but there's no harm in asking nicely.

ASK is also very good for Sennheisers, Sonys and Denons, iirc, and I think you can demo. Friendly staff too.

Tottenham Court Road has a decent selection, I saw, RS2's, HD650's, SR325's, ATH-AD1000's on display. however I couldnt Demo any of them and they were quite pricey.

Audio T in London road (from memory a few shops down from Maplin) stocks at least the lower end grados, and i think they sell senns as well. Got my SR80s from them back in the day.