Assuming that your UK licence and JAA medical are still valid, you will merely need to take a renewal LST with a UK/FE(PPL). Your SEP Class Rating will then be valid for 2 years from the date of passing the test.
The test itself consists of a coiple of steep turns, a couple of stalls, an off-aerodrome PFL plus a couple of circuits. Shouldn't take more than an hour if the FE runs it properly.
The only fees payable will be the cost of a/c hire plus the Examiner's fee (which varies - I charge £77). The Examiner will sign up your licence there and then on the Ratings - Certificate of Renewal page and a form will be sent to the CAA.
There is NO fee payable to the CAA.
You may fly by day, under VFR iin UK airspace in a G-reg a/c on your Canadian licence; insurance will be a matter for the person hiring the a/c to you.