Pretty self explanatory title. Looking over the list of British PMs it was very common for their time in office to last two years or less in many cases. Then when you hit Thatcher that seemed to change and now modern PMs seem to last many more years than in the past. Was there some structural change in the UK parliament that led to this?
Butahime is correct: Gordon Brown was only Prime Minister for 3 years. It should also be borne in mind that Churchill was Prime Minister for 9 years and Harold Wilson for 8 years, just not continuously (they lost power and then regained it later). Wilson could have stayed on for 3 more years, but resigned due to stress and poor health. So both of them were Prime Minister longer than John Major was.
Now, I do think it is noteable that in the modern period parties as a whole have been able to win more elections continuously than in the 40s-70s; Conservatives for 17 years, then Labour for 13 years, and now the Conservatives again for 10-15 years (we'll have to wait and see). But that's a different question (which I'm happy to discuss if you wish).
I will address your subquestion however: no, there has been no structural change in the House of Commons system (I assume that's what you mean). The election system is the same as it was in the 60s and 70s.
Only Blair had an unusually long tenure as PM. One example does not a trend make.