WorldPay actually worked out as cheaper than options from HSBC or Barclays for me. It's still not cheap, and in fact for any credit card transaction over about GBP15.00 PayPal is cheaper. Debit cards are a different matter as they cost a fixed fee per transaction via Worldpay (and I'd much rather everyone paid me with debit cards!). Customer service and response from WorldPay has been good in my experience.
You can reduce the cost of WorldPay if you get the BankDirect (rather than a WorldDirect) service, but you need an internet merchant number from your own bank. I have yet to discover how much my bank will charge for this and whether it actually saves money.
Classic PayPal is convenient for many buyers and costs nothing to set up to take credit cards. The downside is that it is near impossible to talk to anyone in customer services. PayPal Pro offers more functionality but costs. Personally, if you want a low cost option, Classic Paypal is the way to go. A healthy chunk of my sales comes from PayPal transactions.
As for carts, Fighting 15s uses a hosted solution from ekmpowershop.com . It can be set up so that a customer does not have to register. You can play with a working demo online, changing templates, fonts, pictures and so on. It does cost, however, running at around GBP25.00 per month. But then it requires almost no tech knowledge to set up a shop, and it integrates with a number of popular payment systems (not cardnet, though).
For me ekmpowershop was a great step up from a hand-coded site with PayPal payment buttons, plus I could access it anywhere from, importantly for me, a Mac. At the time, however, there weren't the number of free, browser operated shops that now exist.
In short, running an internet shop that takes real-world payment solutions costs! (Exceptions apply to here to anyone who can code their own.)