It may help you when passing your message to say something along the lines of,
"G-****, PA28, passing East abeam somewhere, heading 010 at 2500' on qnh 1025 Student Pilot on NavexVFR, estimate your zone boundry at time three five, with information India, requesting joining instructions."
Now, I know this is probably what it says in CAP 413. And I really, really don't want to confuse bg187. But IMHO, this is too long, and a lot of it is unnecessary.
Passing east abeam somewhere. Fine
Heading 010. Do you really need this? They know your position and that you're inbound. Unless you're making some complicated circuitous route, do they really want to know your heading?
2500' on QNH 1025. I seem to remember some ATCO saying on here that this is pointless. They're probably about to give you their QNH or QFE and tell you what height they want yo at, so is this useful?
Student Pilot on nav ex. Student Pilot will do. They know you're on a nav ex; it's obvious.
Estimating your zome boundary at time three five. OK, but I find it easier to say "in five minutes" and no-one's ever minded.
So the call becomes: G-XXXX, PA28, inbound to you from nowhereville, passing east abeam somewheretown, student pilot, estimate your zone boundary in five minutes, with information India, request joining instructions.
I do it like that, and no-one's ever complained, and if they want more info they ask for it.