You've done well if you've stuck it out this long - there are only 27 clauses so the end is in sight! This is a dreary one, and not really required since the point is already covered by UK law:
19. Value Added Tax
The Publishers require details of the Author's Value Added Tax registration number where applicable. Where the Author fails to provide a Value Added Tax registration number the Publishers cannot pay Value Added Tax on any sums under the terms of this Agreement.
Well, durrr. If you are VAT registered, you already know you have to provide a proper VAT invoice, stating your VAT number, date and tax point, bla bla.
Don't panic about whether you are or should be VAT registered. If you are, you already know. It's not like Income Tax that just sneaks up on you as soon as you have any money. You have to jump through hoops to get VAT registered. You only need to register for VAT if your turnover (not income) goes above £70,000 in a year. You can choose to register with a lower turnover, but if your turnover is very far below Customs and Excise might not want you to. (There used to be a quarterly limit, too, but that is no longer the case.) You can deregister if your turnover drops below £68,000 but there is not point in constantly registering and deregistering if you are hovering around the threshold. We'll do benefits and disadvantages of VAT registration another day - for now, you just need to know if you are VAT registered.
There is nothing to strop about in this clause as UK law already insists you provide your VAT number, so just accept this one and move on...