I found a moneysavingexpert.com site. It is a pro-consumer site for the UK, and seems to work fine with Safari.
I'm not sure what the issue is with Mail, but I found several warnings on the web about scam emails claiming to be from Money Savings Expert. Best practice is to never go to a site through an email link unless you know for certain the email can be trusted. Event then, it won't hurt to go to the site directly in Safari.
Given the large number of email/newsletters available from this site, it would be easy to slip up and click in one that was fake.
Other than that, I'd need more details to guess what is going on. What exactly do you do and see (step by step if possible) and the exact message you get.
Also, I'd like someone else's option (preferably from the UK) on the validity of the site. It seem ok to me, if a bit pushy on the email newsletters, but a local should be able to make a better evaluation. I don't know what is typical for a UK focused site.