There is a section with maps for places to charge bottles on the forum. Maybe if you ring one that's close to you they'll sort it out for you? We use a dive shop in Oakham next to Rutland Water. Probably about a 35 mins drive from St. Neots.
AA S200 .177
AA S400 Carbine
AA S400 MPR FT
AA S200 .22
a bit of a theme emerging here......