OK so first thing's first, I'm no flight planning expert, JUST an ATCO. For Alderney I'd expect to route you via LYD then G27 to NEVIL. This is of course providing you're happy flying the more direct, but longer over-water distance. Else file G27 - Y8 to go LYD, SFD, GWC, SAM (SAM route required to stay inside CAS) - ORTAC.
As was rightly mentioned routing via SAM to HE will leave you outside CAS after SAM. Options would include asking the MIL for a service or perhaps Bournemouth LARS(?). I'm not sure if it's significantly longer on the routing front but another option would be to route via CPT and L9. This would keep you inside CAS for much longer. Be warned that if you wish to enter CAS ASAP after departure from TO you can expect to be routed a long way South unless Gatwick is very quiet. Something like LYD - SFD - GWC - CPT. An option worth thinking about is staying below CAS routing via BIG - OCK with maybe a friendly service from Thames, Gatwick (maybe) then Farnborough who will negotiate directly with TC for joining instructions. You'll save about 40 miles then and still be inside CAS well before CPT.
Hope that helps!