Depends where you are as to which would be best. My guess is that the Southern Half Mil map would be ideal. For 1 1/4 Mil map then probably
Sheet 5, Central England & Wales Chart would be useful. There's several official stockists (
Stockists | VFR & Chart Information | Airspace Policy) or you may find that a local flying club at an airfield will have them for sale. Your own club may even have them. The maps are about £16.00 each (get the laminated ones so you can write on them with dry marker or chinagraph pens).