When you come back from Laos your SETV will get you in for 60day, when you go back to UK whether in the first 60 (or subsequent 30 extension) you'll need a reentry permit (from airport or local immigration office) to keep the visa and it's permitted to stay date.
Elizabeth - just a note its not 'visa on arrival', you'll be 'visa exempt'. Quite different things. Other than that you should be OK. You could check with the airline.