If you're only going to need it for 3 days, I'd find one in the UK that he likes and bring over a tin of the powder. That way you'll know he actually likes it and that's one less thing to worry about. I've heard of babies being sensitive to the taste of the water used to mix up the formula, but I'd say that's rare. Mixing it with the breast milk to help with the transition is a good idea. For those three days, if you have enough breast milk (and if he has already proven he likes it), you could ask that they mix it with the formula (3 oz BM, 2 oz formula per bottle for example). I express milk for my son, but occasionally I've had to top up a bottle with some formula.