Worse that can happen if you print out a business card and try it on is they say no, in which case just try another store, once you get issued it in one store it works in all of them anyway, lol

If my mrs signs anyone up she always asks to see either trade insurance or certificated proof of either qualifications or employment in the trade I beleive so probably wouldnt take any notice of a business card on its own, but im sure some of the people signing you up will do so as my mate did it a couple of years ago in the uxbridge store, he just printed off some cards and headed note paper and took it in and they gave him the card no argument at all.

Its all down to the member of staff you happen to talk to I think!