I hope someone can give you some pointers but I would urge you and your wife to take some "heavyweight" personal tax/financial advice. Will your wife be tax resident only in Belgium or also in the UK. What about you? There are rules to prevent "double taxation" but someone needs to know what action needs to be taken to ensure liabilities are correctly calculated and reported, and any reliefs claimed.
What is the impact of all of this on any state and private pension provision (in both countries)?
I would be very surprised in the recruiting company doesn't have a firm of accountants that it uses to assist senior people - especially with regard to those moving cross border. It's possible that, if your wife takes the job, they might provide something like that as a benefit in kind.