Danesfield House is near Henley which is a lovely town to look around with a choice of places to eat which keeps costs down as hotel restaurants are a bit costly to eat in every night.
When choosing a spa hotel I have two tips. Before you book call up the spa and find out what the spa is made up of, as in what is included which doesn't require extra money. Usually you get pool and sauna, but find out if they have a jacuzzi, ice room, jet pool, steam room, outdoor pool etc which makes it more fun. Some just have the basics which means you have to spend loads to get much from it.
Secondly once you've booked the hotel, call the spa and book any treatments straight away so you can have them when you want rather than just squeezing them in.
If you are around the Henley area and don't want to travel but have some cash, take a look at Cliveden, it has a nice spa, not loads going on, just a pool and saunas but has lovely hot tubs outside which are these cool wooden ones. They also have an outdoor pool, all enclosed in a beautiful walled garden.
They have a super posh restaurant but the saloon style club house is a lovely place to grab a club sandwich or a simple hot meal. So cool, the bar stools are old saddles.
Lovely gardens too as national trust.