Well spotted - a well-known 'ruse' to those of us in the SayNoTo0870 community thanks.
It's worth remembering though that use of a 141 prefix is usually prudent when using such numbers because some organisations (Alliance & Leicester are a prime and shameless example) refuse to talk to you if your CLI shows a UK call origin.
As a point of interest, both my Conran VISA and Morgan Stanley Mastercard credit cards (as recommended on the
MoneySavingExpert.com site) not only give 1% cashback but also have 0800 Customer Service numbers!