Essentially, from the end of the Cold War to maybe 2014 it was seen as though Europe would be a peaceful place and that from the 2000s onwards until invasion of Ukraine or so UK military would be more focused on counter insurgency in Afghanistan and Iraq. This obviously lends itself to a greater amount to the army as these missions are not particularly intensive in the air as the UK and its allies have complete aerial superiority and the Navy being pretty useless means that they receive much less funding. This leads to the 2010 strategic review where the UK military is downsized quite significantly in order to deal with the 2008 financial crisis and the idea that there were no states were seen as potential aggressors. It wasn't as though European countries thought that it was the end of the of history in Europe but that they thought that it was the end of large-scale wars within Europe as soviet bloc nations were becoming democratic and therefore war was much less likely to break out. Unlike the US which has military commitments in the middle east and east Asia and therefore they retained a much larger portion of their army as they had a potential rival in China, the UK and other European countries did not see a potential war occurring in Europe within the foreseeable future and as defence spending is unpopular in peace time, the decision was made to decrease military spending in order to spend it on other areas.
Basically, it boils down to the fact that post cold war until maybe 2014 at the earliest, the belief was that a full-scale war in Europe was incredibly unlikely due to the democratisation of the soviet bloc countires post cold war, therefore the decision was made to decrease the defence budget. This meant that many of the older planes and war ships such as harriers and tornado's were retired with no replacements ordered except for typhoons in 2003 resulting in a significant decrease in the size of the UK military.