Having a bespoke e-commerce system written from the ground up would be terribly expensive and I'd avoid that as a possible solution like the plague. It will also be completely unnecessary since there are so many mature and readily available systems.
There are a great many methods of processing e-commerce transactions and without knowing the scope of your intended operation it is impossible to suggest one method or product over another.
Speak with your bank and your web host first. Many banks have off the peg systems and web hosts sometimes offer one or more options for trading online as well.
Some people report success with the NOCHEXS system, but I can't recommend it as ideal unless I have an idea of what it is you want to trade in.
Take a look at the UK Web Design Association site click here
It is searchable by location and you should be able to find a local web designer who will be able to help.
Failing that, provide some more detail and we may be able to nudge you in the right direction.