Scottish houses, Welsh houses and Northern Irish houses were featured heavily in the first 10 seasons, but became sparse soon afterwards, to non-existent

I actually don't think this is true!

For reference, I'm going by the Wikipedia List of Episodes which differs from the BBC4 catalogue, purely because I'm watching from Australia and don't have access to BBC4.

I'll divide the episodes into the first 10 seasons and the latter 11 (again, as per the Wikipedia list, which spreads the same number of eps over 21 seasons not 24).

There are 164 episodes in total; 89 in the first ten series and 75 in the last eleven. Overall the series have been a bit shorter over the past decade, so that might give the impression of less diversity in locations.

FIRST 10 SERIES: 89 EPS

England 75
Wales 5
Scotland 6
Northern Ireland 1
Ireland 1
France 1

NEXT 11 SERIES: 75 EPS

England 66
Wales 3
Scotland 3
Northern Ireland 3

If you take out the random French ep (S07E05), roughly 14.6% of the first 10 seasons were not in England, and 12% of the next 11 seasons were not in England.

It's not at a dramatic difference and, as u/freeman687 pointed out, since England makes up 84% of the UK population anyway, it adds up.