I'd suggest the Victoria and Albert Museum in South Kensington. Although there isn't a lot that is a concentrated collection of just graphic design there are lots of great examples of it amongst other good design in the modern galleries. I love the fashion collection and the S.E.Asian collections. 19th Century plaster casts of the Trajan Column and similar monuments displayed in a 19th Century manner. Lovely building. Good bookshop and gift shop. It's free although there are some exhibitions that have an entrance fee.
http://www.vam.ac.uk/

There's a Wim Crouwel exhibition at the Design Museum while you're here.
http://designmuseum.org/exhibitions/2011/wim-crouwel

I'm sure there are many design studios in London, but one that pops into my mind is the London office of Pentagram (pentagram.com), a famous international graphic design group. Obviously you'd need to contact them ahead of time to see if they'd have time to talk to your students.

http://typophile.com/node/35832

The London Transport Museum is good for graphics, from posters to the lettering and ads on vehicles.
And of course the famous Underground map.

• the Underground
• shop front lettering
• St Bride library (a MUST for typophiles)
• Westminster Abbey (epigraphy)

Victoria & Albert and Transport museums are excellent suggestions.
I always get as much inspired by the breathtaking antiquities collections in the Britisch Museum.
And if some pounds are left, embark on the “London Eye”. It’s worth the money.
.

Banksy.

Check out what exhibitions are on at the British Library during your visit. The permanent exhibit of books and manuscripts is always worth a visit, and there are often interesting temporary exhibits.

Thanks for all the great ideas!

Always look up! There is carved lettering to be found all over town.

Head to Brick Lane and up towards Old Street (East End) for graffiti and street art...don't venture too far off the beaten track though...ride any of the overground trains around the south and you'll see some pieces on the warehouses and retaining walls.

The Cemeteries near Highgate might offer some rewards too.

@1985 - good call. Always be looking up, the commercial signage for Chicken Cottage et al is a feature in itself.

I'd split them into groups, it's a big ass city to cover.

street art, the british library permanent exhibition, and maybe the odd people of the last tuesday society: http://thehendrickslectureseries.co.uk/lectures.html

@ _leigh

...don't venture too far off the beaten track though..

Potential worst case scenario these days is ending up at the Olympic site!

@1985 Heh, even Brixton is calm these days.