For testing I use Parwin Scuba at Manor Farm, Glatton, Huntingdon PE28 5RR.
Closed Mon and Tues. Open Wed, Thurs, Fr, Sat 08.00 - 16.00; Sun 08.00 - 12.00 Do not do 'while you wait' as the owner lets bottles cool down while filling.
They have a web site.
For fills only I use The Country Store, Pampisford, near Sawston, Cambridge. Can do a 'while you wait' fill.
Cheers, Phil