Hi Rackatt
Well there's quite a few broadband ISP's who would suit your needs really.
If it's a basic service you need I would seriously consider using Zen Light from Zen Internet - for £15.31 a month you get an 8Mb, 2Gb small business broadband connection for one PC, they'll come in and set it up for you and if you need any additional tech support they can come out and see you as well. They have a very good reputation for customer service.
Others to consider are Demon, Bulldog and BT Business Broadband.
In terms of whether it will interfere with your switch/credit card machine to be honest I'm not enough of a techie to give you a reliable answer - the ISP you use should be able to tell but I do doubt it would be affected.
Kind Regards
Jason