Forgive me for not really answering the question..
Following on from Chilli Monster
Transition Level is a flight level (IE FL45, FL60 etc) and Transition Altitude is a "height" (in speech marks due to the books use of height, level and altitude). To me, this means that if an aircraft was climbing, 1013 would be set on the altimeter once through 3000'; on descending, regional QNH would be set once through FL45. (as Aussie Andy alluded to)
The pressure in the UK rarely goes below (or near) 950 millibars, or above (or even close to) 1050 millibars. The usage of those altitudes therefore allows for 1,500 feet
1013-950 = 63
1050-1013= 37
At 30 feet per millibar, that'll give 1890 feet tops. 1500 is good enough as we don't often see those extremes.
FFW