I would also recommend litecommerce. It is a easy to use package and has the UK tax system covered. It is made by the same people as xcart. It is NOT suitable for large (over 2,000) products as it gets slow.
I find that if you like to tinker and amend and heavily change a cart then xcart is better, but if you want a package that does most of what you want then Litecommerce is better.
That said you have not said what you want to sell, how many products you envisage, how many categories, what design control you want etc etc etc. All of which will have an effect on what cart is best FOR YOU. We are just saying whats best for US.