It is possible, ask Wycombe tower to co-ord it for you. You may find that you have to hold outside at Cookham initally. It will also depend on the runways in use at the time - both at Heathrow and Northolt.
Alt, you can ask Wycombe tower for a contact number and talk to "Special" direct
Then there is always Northolt Approach (126.450) who also provide SVFR in that area of the zone...