I doubt you would want to fly "in formation".
Formation flying is a definite skill and certainly not one to expereince with another pilot who by the sound of it might be a bit of a "novice".
Far better, fly "in company with".
Put more simply this is a very lose formation of you and another, in which you take the lead and he follows.
You would simply tell AT that you are G-XXXX in company with G-XXXX another C152 or whatever.
Expect air traffic to tell the other aircraft to switch their transponder to standby.
When you arrive at your destination you might want to sign of the company call with G-XXXA breaking company with G-XXXB and calling for joining instructions. G-XXXB can then call for instructions in turn.
You might also want to think about whether you are going to agree an air to air frequency and what you will do if for any reason you get seperated.
Also you will need to give your friend some advice on how to maintain the company. For example the cruise speed you will use and the position he should adopt relative to you.
Formation flying is great fun but at least take some tips on here from people who have done it before if that is really what you want to do.