Approach/Radar, fits in with approaching the field. Also Approach is appropriate for calling at long distance for airfield info (QFE, QNH R/W etc) as the approach freq is chosen for a larger area of none conflict with others on same freq. Tower freq for circuits. If there's an ATIS then listen out as it may give a different freq in use. This can happen if it's quiet (mid week say) when all work is done on one freq, with two freqs twr and app at busy times.
If you call on the approach and then would rather you call tower, then they'll politely (at most fields, and I understand Blackpool to be one) ask you to change.
Don't be frightened by the radio, most ATC will treat you gently, especially if you prefix 'tyro' with the military ATC then they know not to overload you.