I get you, OP, and I'd feel the same way in your circumstances. It's about being actively valued, appreciated and loved on a daily basis. It doesn't have to involve lots of money, but it does need to involve being celebrated for being you. Sex I could get over: I've never really understood those women who think that one drunk kiss at a Christmas party is the end of their marriage. A £20k diamond necklace is a serious gift. It carries a message that a nice practical middle-range jacket doesn't. Gifts are symbolic. He knows this, you know this.
Your DH seems, from what you've said, to hold you in the practical-sensible-mother-of-his-children category. The OW was in the romantic 'I-love-you-to-the-point-of-extravagance' category. Classic mistress territory. Exciting fantasy time, whereas you and the children are dull reality. This is a betrayal of your children as much as you. He took the risk of upending their lives as well as yours.
It's clear that you have a very comfortable life. You don't mention whether you work or not. Such a financially comfortable position can be very difficult to give up. Tempting as it may be to soldier on, in your shoes I'd have to leave, because this new discovery shows that
a) he's not been transparent and contrite and come clean: you've had to dig to get any idea of what he's been up to. Who knows what else he's successfully buried? And
b) he has a fundamental lack of respect for you beyond the fact that you're the mother of his children. What's going to happen in a few years when those children grow up and leave home? We probably all know people left later in life in empty marriages where there's no real feeling involved, or women dumped in their 50s for a younger model.
Personally, I'd prefer to take back control and preserve my dignity and self-respect by putting an end to it now. It sounds as if you now have all his financial paperwork, although given the way he's behaved I wouldn't necessarily trust that he hasn't salted assets away where you can't find them. In your shoes I'd want to investigate further before quietly consulting a good, aggressive divorce lawyer and taking him to the cleaners. I really can't imagine wanting to stay with a man who saw me as 'just' his wife and who gave £20k diamonds to his mistress and didn't apparently, have any remorse for it.
Get away, create a new life for yourself, find someone who doesn't just see you as useful. Good luck.