Cannot tell precisely about the UK but it depends a bit from country to country, whether they "accept" the 6 digitcode.

For example in Denmark, France, Germany, and Lithuania there are no problems with 6 digits, while in India / Nepal they would not accept it (luckily I changed mine to a 4 digit).

Would recommend that you change yours to 4 digit (unless you are worried about security) - then you will be on the safe side!

Forgot to mention that we tried with Maestro, Postcard, and Visa.