Which means that you will lose out if the CAA doesn't return your rating validity page promptly. So always make it plain that 'time is of the essence' in the contract you enter into with the examiner who is acting on behalf of the authority - and indicate as such in a covering letter in your paperwork.
The 'valid from date of issue' rule is one thing - but as you can't actually use your rating until you get it back from the Belgrano, it seems unreasonable to date it from the date of the flight test. In my opinion it should be from the date your rating was approved at Gatwick.