I immediately thought of the Celtic Manor too. It is on the outskirts of Newport, South Wales, but close to the English border. You would just need to pop down the M5, and would have 3 main choices of route. The first one from you means that you exit the M5 (just after the Strensham Services - I think, but please check as I have only used that route once), you then get a slower, but more scenic route through the South East corner of Wales, maybe through the very pretty Wye Valley; or you can continue on the M5 southwards, and then where the M5 & the M4 meet you turn right for Wales, go over the Old Severn Bridge to Chepstow, then make your way further South to Newport, which is a shorter scenic route than the previous one; or if you want to go on the fastest route, you can carry on, on the M5 until you see the signposts for the New Severn Bridge, and when you are on the other side of that it is just a short journey on the M4 to the hotel.
The Hotel has a large indoor Swimming Pool, and luxury spa treatments in the main Hotel complex, a well equipped Gym, and in the grounds there is one of those tree-top adventure thingys. There is at least one other indoor pool on the larger site, but I have never seen it. I have one word of caution though, if any of your family members love playing golf, and you don't, you might not see them for much of your time away, as it is also a major golf resort - it hosted one of the biggest golfing events in the golfing calendar some years ago, but the main hotel is not usually overrun by golfers, as the nineteenth hole is in another building.
The main building is quite modern, the older 'manor house' is much smaller, but you have to go to the larger one for the swimming pool and gym etc. If you do decide to go to the Celtic Manor, don't let them "upgrade" you to the older Manor House - as from what others have said, it isn't an upgrade at all, the rooms are apparently not as nice as the main building's ones.
There are a lot of different grades of rooms, and several hotels in their portfolio now, so first of all join their mailing list online, as they quite frequently do some good offers. Then browse through all the info online before deciding whether it seems right for you and your family, some things may have changed since I was last there, as our circumstances have changed and we cannot really afford it anymore. Anyway, wherever you end up going for your birthday treat OP, I hope you have a lovely time, and enjoy being 40, my next major birthday is my 70th (God Willing)!
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