There's a few specialist spa booking companies out there, who seem to have good all-inclusive rates. Those can usually be beaten by ringing the hotel and asking them to match, but their websites are often a good initial point to start searching from
Using points wise, the few I've tried lately:
We stayed at the Hilton Doubletree Dunblane Hydro last week, which is in the middle of Scotland but not near Essex... Even peak summer weekends it was only £150/night for room + dinner + breakfast. They also had packages with spa treatments included. The possible downside is that it's an old Victorian spa hotel, lovely and grand, but the current spa facilities are in a much less glamorous building next door, and certainly not up to fancy spa levels!
Also up North, the Sheraton Grand in Edinburgh has a huge spa and leisure area, but the hotel's bang in the city centre (good normally, but not for you!), and I've never felt all that relaxed in the spa / pool area though it's large.
The Crowne Plaza at Heythrop Park in Oxfordshire is another spa-lite spa-ish hotel we've enjoyed a weekend at fairly recently. Spa stuff is in the gold club adjacent, decent for a normal hotel, not super-lux spa levels though. Out of the way on a lovely big estate, loads of nice countryside walks, decent food, and just-about reachable on public transport...
IC Marseille is lovely, very nice spa and great food, just hop on the direct Eurostar down there
