Hi -
You will indeed need a class 1 medical before you commit to a fATPL course however I would advise leaving it as late as you can. The class 1 medical costs just over £300
Link to CAA
and is only valid for a year..
Validity link
With that said - the renewal is not as bad cost wise.
If you have less than 60 hours flying experience (or no ppl), there is a fully funded fATPL up for grabs (im sure someone can point you at the link). Alternatively you could look at the FlyBE part sponsored flight instructors scheme...
Cabair link
just a couple of ideas..
Steve