if you want to be pedantic - the UK is a series of Islands, the only land border we share with another state is Ireland. not only is the UK Irelands largest trading partner, but its offensive military capbility is not far off zero. not only that, but there are probably 3 states in the entire world with the amphibious capability to launch any kind of invasion of the UK, and all of them are our close allies. in which case, why would we possibly want 'Regiments of Troops' for defence?
if you wanted to design a purely defensive force for the UK, you probably take the Army down to around 10,000 men and spend the rest on air and sea power (remembering that the UK imports half its food and vastly more than half its oil and gas...). unfortunately, air and sea power is also easy to use offensively...