Hi KateRVN
Comming off IR ESA and going on to UC can be quite painless providing you have done your homework before you apply for UC.
The first thing to remember is not to cancel your ESA before you apply for UC not even for 1 day, if you do then you will have to go through the assessment phase.
The second thing to do is seek advice from your local Welfare Rights Organisation before you apply for UC to see if you will be better off or worse off, quite often claimants who are not in receipt of PIP will be better off, those in receipt of PIP will loose any disability premiums and quite often they will be worse off so seek advice first.
You can find your local Welfare Rights Organisation to help you with advice by clicking on the link below and putting in your post code, it will then come up with a MAP with a list of agencies.
advicelocal.uk Gary