Some good advice above!
Popham-Cardiff -now THERE is an interesting trip. W/e's I'd go direct track after checking D128 closed, but u may wish to route via Stonehenge for some interest(talk to Thruxton!)Also the route down the A303 is good. Weekdays I'd be tempted to get an overhead at Boscombe (usually possible, but they may be busy at the moment!) Sometimes they offer a low level route (not > 500' down the railway line -that's fun!) Then talk to Yeovilton for a LARS. On initial call, callsign, then ASK for LARS and FIS, then they will know exactly what u want.
Have fun!