LONDON...ooh, and probably the rest of the UK, e.g. Durham, Oxford, Cambridge, Bath, perhaps also Scotland too. I'd go mental going round all the museums (I simply have to go to Apsley House and the Royal Opera House), theatres, sites, cathedrals, second hand bookshops (people are probably going to go 'WTF?' at the last place) etc. Then a massive book/souvenir shopping spree will ensue methinks. XDDD
I'd probably take my mum but she's technically more like family though I would really consider her my friend too. I'd go with enserric but I'm not sure how that will work seeing that she's already in the UK!