Legally for a FIS/A/g service all you need to do is be in radio contact. No clearance is required (or possible - they don't have the authority).
In practice I wouldn't give the full call, just
'G-ABCD is a XXXX 5 miles southwest at 1700 on 1014, intend to transit your overhead at 1700ft, request FIS [if it's a FISO] and QNH'
You need the QNH so you are on the same reference as anyone else in the ATZ. You might add 'expecting overhead at 21' but to be honest if you've given your range and aircraft type most people know roughly when to expect you!
Tim