IIRC the British only sent their expeditionary force to the Peninsula in 1809.
Summer 1808. There was a small chance of an engagement between Napoleon and the British in the winter of 1808 when Sir John Moore marched his army against Napoleon's lines of communication in order to disrupt his invasion of southern Spain. Napoleon managed to turn his men around much faster than anyone expected and almost trapped the British, but Moore saw the threat in time and managed to evade the trap.
RE Napoleon landing and facing regular British regiments...just how good would the average British regiment be? I mean assuming a landing within a year of an altered Trafalgar, will Britain actually have enough hardened regiments on hand? It's not as if Napoleon is going to be facing a magically ISOTed Wellington complete with the Peninsular Army from 1811 in OTL. I'm guessing that at this point, the only real veteran troops the British would have would be those who had served in India.
There were
66,000 regular troops in England in December 1805, and another 100,000 or so militia (the militia were trained and equipped up to regular standards, but not liable for overseas service) plus a hotch-potch of up to half a million men in other volunteer and part time units whose quality was rather more variable.
As for quality - well, the bad years of the 1790's are well past by any likely invasion date and the British army had worked out how to beat the French at least as early as 1801 (Battle of Alexandria) and it didn't take the Duke of Wellington to do it either.
Of course, as Thande has pointed out, having better troops won't matter if Napoleon can't resupply them. Southern England might end up sacked and raped but Bonaparte will be worn down.
Exactly, and this was pretty much the calculation of the British government of the day.
The question is if a landing in force will actually scare Britain into seeking terms. If Napoleon is at the gates of London will the British sue for peace or fall back and fight on?
The plan was certainly to fight on. British preparations in the event of invasion were actually very advanced, and had even gone so far as preparing the village of Weedon in Northamptonshire to act as a back up capital in the event of London becoming unsafe.
Of course the biggest problem with all PODs of this sort is that if resources are being spent on giving the French fleet the capabilities to crush the Royal Navy or to force the Channel for long enough to land an army, those resources aren't being spent on the Army.
Well, quite. A point that can't be stressed often enough. What are Austria and Prussia doing while Napoleon is running down the army in order to build a fleet capable of taking on the Royal Navy?