The ESTA is valid for two years. However, the visit duration is maxed at 90 days, and does not reset if you visit Canada or Mexico - you have to leave North America to reset it.
The only problem with returning again 'soon' is that there may be additional questions to ensure you're not trying to re-enter to work or stay. But as long as you have the documentation to confirm this - your return flight to the UK, accommodation details, details of where you work/study back home, you will be fine.
Source: I'm on my second ESTA period now, and have been to the USA several times during that time, and lived in Canada for an overlap of the two ESTA, so I've been through this a lot.