Thank you all for your replies.
Ukreg.com is supposed to be a legit site. Actually it is operated by fasthosts.co.uk which is one of the biggest web hosting companies in UK.
As for that phone call... message, I was surprised, too. What kind of company makes phone calls to every soon-to-be-registered customer for privacy reasons, these days?!?!
Moreover, what kind of company promises they will call you within 15 minutes?
I've just checked the whois for the domain, but it still seems available.
I've used my Switch card to buy the domain, and the card is 100% ok and valid.
I've also managed to find a "sales inquiries" email form, and contacted them but still no answer from there, too.
If you go to the company (fasthosts.co.uk) that operates UKreg.com, you still have to have a username and pin number to enter the support area.
So it seems that for the time being there is nothing I can do.
I'm thinking of waiting until Monday and then I will contact my bank to see if ukreg.com have taken the money out from my account.
UPDATED:
________
I finally managed to find a contact phone number of the parent company at this site : (I was lucky)
http://www.dslreports.com/forum/remark,9214188
so I called the parent company and I explained the situation.
I was told that they tried to call me and I didn't answer (what?!?! - I had given them my mobile and my home number, and did not receive any call at all by them).
Anyway, they told me that no money were taken out of my bank account, so I went and registered the domain name I wanted at 123-reg.co.uk.
Everything seems fine this time and the only thing left is to see when the domain will show as registered at web whois (probably within the next 24-72 hours).