thankyou all for the advice.
My strategy has changed. We're going for a wander somewhere close to home in August (either Essex and Suffolk or around Dieppe) and that will sort out any problems we have with the touring routine - I'm aiming to keep the two bikes and luggage to under forty pounds, which means we'll be travelling with a minimum amount of stuff, so it will be good to test out the practicality of this. Then, having had a wonderful time (fingers crossed) we'll aim to do the Outer Hebrides in late September or mid October, or, if the weather is rubbish, May next year.
I think aiming for everything at once on a first tour might make the first tour the last tour. And that would be sad.