I copy below a list of hotels from an old magazine cutting entitled "Escape - Spa Special". I think all or most of these are spa hotels, but I think they are also very expensive (I have not tried them):-
Cedar Falls, Bishop's Lydeard, Somerset; 01823 - 433233. Champneys, Tring, Hertfordshire tel. 01442 - 291111. Chewton Glen, New Milton, Hampshire, tel. 01425 - 275341. Bath Spa Hotel, Bath tel. 01225 - 444424. Grahshott Hall, Grayshott, Surrey tel. 01428 - 604331. Hanbury Manor, Ware, Hertfordshire, tel. 01920 - 487722. Henlow Grange, Henlow, Bedfordshire tel. 01462 - 811111. Hoar Cross Hall, Hoar Cross, Staffordshire tel. 01283 - 575671. La Fontana at Sprowston Manor Hotel, Norwich, Norfolk tel. 01603 - 410871. Le Petit Spa, Malmaison Manchester tel. 0161 - 278 1000 and Malmaison Newcastle tel. 0191 - 245 5000. The Lute Player, East Malling, Kent tel. 01622 - 817300. Nirvana Spa, Sindlesham, Berkshire tel. 0118 - 977 1777. Ragdale Hall, Ragdale, Leicester tel. 0116 - 443 4831. Runnnymede Hotel and Spa, Egham, Surrey tel. 01784 - 436171. The Spa at Carden, Near Chester, Cheshire tel. 01829 - 731000. Lucknam Park, Colerne, Wiltshire tel. 01225 - 742777. The Spa at the Globe Hotel, Surbiton, Surrey tel. 020 - 8339 9754. The Spread Eagle Hotel and Health Spa, Midhurst, Susssex tel. 01730 - 816911. Springs Hydro, Packington, Leicestershire tel. 01530 - 273873. St. George Hotel, Harrogate, North Yorkshire tel. 01423 - 561431. Utopia Spa at Rowhill Grange, Dartford, Kent tel. 01322 - 615136.
The list included some hotels in Wales and Scotland but I have not copied them as you specified they had to be in England. Also: I tried to type all of the numbers correctly but a few mistakes may have slipped through... Good luck.