The best-known supplier of bookkeeping software is Sage. Their principal products are Sage Line 50 for medium businesses, and Sage Line 100 for large businesses. For small businesses there is Sage Instant Accounts which is a cut-down version of Line 50. There is also Sage Payroll for dealing with wages and salaries.
If you adopt Sage products, then you are likely to find that most professional accountants can give you good support, since it is the system that we all grew up with. Some businesses employ a bookkeeper to manage their Sage system.
You can obtain Sage from most accountants, or from shops such as PC World or Staples UK. It is standalone software, which means that you buy one copy for one computer.