Best of luck. Given the riding you're already doing I think you'll find that you surpass your expectations for how much you can ride each day, if you want to. You may wish to (plan to) ride longer on some days, for example where the scenery is urban (eg Runcorn to Preston, Hamilton/Glasgow/Dumbarton).
Some route framework advice:
https://www.cyclechat.net/threads/lejog-or-jogle-east-or-west-coast-routes.233566/
Notwithstanding your Lizard Point to Dunnet Head termini, I'd recommend you start at Land's End and finish at JoG (though yours is no less an end-to-end) to tick both boxes. LE to the Lizard (there's a YH there) is a day's ride (with your schedule) and Dunnet Head to JoG is 15 miles. Or start at the Lizard and go via LE! I found the (non YHA hostel at Trevescan excellent:
http://www.landsendholidays.co.uk/
As far as off-road routes are concerned these are the south west routes I'd try to weave in (and I have cycled all these routes several times (each) on a road bike (28mm tyres):
If you go north from Hayle and along the coast road to Portreath (YH just south of Portreath in Illogan) you can take the
cycle path to Scorrier.
Well worth getting up to Padstow and then taking the
Camel Trail SE through Wadebridge to Bodmin (NW edge) and then turn NE up the Camel valley heading for Davidstow before going north of Bodmin Moor.
Across on the edge of Dartmoor, the
Granite Way goes over the Meldon viaduct and terminates above Okehampton.
If you stay north (NE from Davidstow, though, then head towards Great Torrington and turn north onto the
Tarka Trail which heads north to Bideford and round to Barnstaple - all well surfaced old railway track. It goes on up to Braunton so you can do the north Devon coast (there's a short stretch of old railway track dropping down in Ilfracombe). If you do go north of Exmoor then take the toll road down into Porlock (excellent surface and virtually no traffic (£1)).
Otherwise Exmoor offers hills after Barnstaple, if so desired. Otherwise good roads to Bampton or Tiverton.)
The
Grand Western canal towpath runs from Tiverton most of the way to Wellington.
Going through Taunton you can pick up the canal tow path and minor roads (which is an NCN route) - taking you through to the pinch point called Bridgwater.
The Strawberry Line is a mostly off-road cycle path from the gap in the Mendips just N of Axbridge, and takes you all the way to Yatton. From there there's an excellent NCN link (26 then 410) to the Avon Bridge and beyond.
Once over the Avon Bridge I'd avoid the ghastly NCN route through industrial/commercial Avonmouth but head up to Blaise castle and then NW down through Hallen and back roads to the Severn Bridge.
There's a cycle path on both sides of the Severn Bridge.
Reaching well beyond the South West, if you go through Fort William and up the Great Glen then the towpath fro Corpach to Gairlochy is excellent, followed by a climb up to the Commando memorial above Spean Bridge (with Ben Nevis resplendent in the sunset (end Day 7).
View attachment 449634
I thought that the Scottish Islands were worth visiting and crossed to Arran from Ardrossan, then ferry from Lochranza (YH) to Kintyre, and then across to Mull from Oban and back onto the mainland (Fishnish/Lochaline). If I'd had another half day I'd have gone on to Tobermory (YH), across to Kilchoan and taken in Ardnamurchan Point (the most westerly point of the British mainland). Beyond my timeframe also was to head for Mallaig and the ferry to Skye.
One bit of advice which may be obvious to you is that when you choose where to stop, bear in mind that you'll want to eat. If you're carrying (the weight of) cooking stuff, fine but otherwise it's good to have somewhere (warm) to eat (heartily) close by. If that's impractical then an early evening fish and chip stop, before finishing that day's ride will fuel you for the next day (and the pints in the pub).
Given that you're riding in May, let me share this link to an authoritative paper on prevailing winds.
https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/wea.301
"The main conclusion to be drawn is that in
Britain, northeasterly winds are at least as
common in spring as southwesterly winds,
and in some years are considerably more so.
Furthermore those years with a low frequency
of northeasterly winds in May are slightly
more likely to precede a warmer summer
than usual."