how much are you anticipating to hire each bunk for? I know of a trade supplier that can do the cheapest for �49.50+vat each, you just need to set up a trade account:
http://www.seconique.co.uk/product.php?id=392& product_id=1&start_from=0
lots of furniture package companies can hire furniture, but of course this is a lot more costly than buying it outright (you're returning it to them second hand obviously). as you'd be buying them trade, you could always sell them for probably what you bought them for (if not more).
if you're adament on hiring, go to www.landlordzone.co.uk and select 'furniture and furnishings' and just work the list it gives you - the majority of furniture package companies offer it, but as there's a risk involved for them, they have to make sure they've covering their costs within the first couple of payments and the rest is profit!