Ok I this is what I know.
I am nearly finished setting up my website that takes online transactions. So I had to get a merchant number. I am going to use a secure third party provider to deal with all my transactions. (try www.protx.com
they are some of the best around.)
With a online merchant number every bank has to go through a process to see whether or not the company/individual is going to make sufficent money on the internet using online transactions.
With every transaction the bank will charge you depending on the card payment e.g Credit Card or Debit Card and a setup fee that can range from £150 - £300.
You do not have to have get a merchant number from the same bank as where your account is held. This gives you a lot more freedom of who you want to choose, but every banks security is different from others.
One that I think you might be able to get is a HSBC one. I recommend you go for the "telephone orders". You can take customers details over the phone type them direct to HSBC over the internet using their secure servers, they check to see if the card is fraudulent there and then. You then receive the money in 3 days time.
All customers details are also stored in your private area so that you can look back at previous clients.
P.S. If you need any more information let me know and i will try to help you in anyway