living with the bf and renting a flat

Quick question- my boyfriend and I are planning on living together when I go over to England for my working holiday next August/September- he'll have just graduated from uni and (hopefully!) have found a graduate job...if he finds a flat for us in June or July and then I come over a few months later, how exactly can I get on the lease as a joint tenant?  Can you convert a single tenancy into joint, or will they even let me do it since I'm coming over with no UK credit history?  Sorry, just a little confused on how all this works... 

Depends on who you're renting through -- DH was renting privately when I moved over, so his landlady just added my name to the lease.  If he's renting through an agency, they'll probably charge you to redo the lease in two names.  I was only here for about six months when we moved and had to go through an agency.  I wasn't working and had only opened one catalogue account and we didn't have any trouble having my name put on the lease alongside DH's.

Most tenancy agreements here from private landlords are only for six months.  They're called 'short assured tenancy agreements' rather than 'leases'.  So it might not be worth becoming a joint tenant if your bf is near the end of a six-month term.

not an expert, but my brit bf wrote his landlord co. (paramount homes) and asked permission for me to move in, not put me on the lease. They wrote back and it's fine. I needed official permission for my student visa - I'm here and no problems.

Can you convert a single tenancy into joint, or will they even let me do it since I'm coming over with no UK credit history? 

I have a slightly different story than yours, but some of this might be of help.... I moved in with my bf when I first came over as a student and we managed to rent a house together with a bit of effort on my part.... They actually wouldn't let me live there without being named on the lease, so getting it in only his name wasn't an option. Also, you'll find that being on the lease, and therefore the council tax bill, will be an enormous help when you're trying to get bank accounts, etc and need proof of address. With no credit history and no job you'll likely have to have a guarantor for your part of the rent. It can be someone in the US, but the agency will need paperwork from them and the time difference slows things down a little....

Oh, and Lola's right that a rental agency is probably going to charge you to re-do the lease - we have to pay £50 to renew ours, so I should think it would be a similar fee...