verasgirl: Can anyone advise what we should do?
First of all don't be bitter, take it on the chin. Credit is not guaranteed.
In hindsight its probably a predicatable outcome. To put a harsh slant on this I think what you wanted was to BT the £2000 to the new Santander card at 0% for 12 months (3% fee probably). Unfortunately thanks (or no thanks) to Credit Reference Agency enhanced reporting the lender can infer from the search that that is your likely intention. Santander decided they wouldn't make enough money from you for the risk involved.
Your ("excellent") Equifax credit score is only one aspect of the assessment. It takes no account of income. Your income (I assume pension) is first mentioned on the application form. Almost certainly the lender has decided that your existing credit balance (and available credit) has reached their limit for your income.
OK, what to do. The £2000 is a done deal unfortunately. If what you want is another credit card then pay down the £2000 from savings and wait a month after your statement. Alternatively pay it off significantly more than minimum payments for 6 months. Then apply for another card which doesn't feature such a generous BT offer.
Alternatively, a prepaid € currency card would be ideal for your travel.
If you can't pay down the £2000 from savings then unfortunately Santander were right.