I’m guessing given the dates that this is threshold sports’ Ride Across Britain. I did it in 2013 (John O’Groats to Land’s End) and last year (Land’s End to John O’Groats).
The new route through the Cairngorms is massively harder than the old route that went over Glen Coe and had a far higher failure rate (of people getting swept during individual stages) than previous years. I cannot emphasise enough how much you will benefit from getting out on the road and doing as much climbing as you can possibly fit in. Hill repeats are your friend and indoor work really doesn’t replicate the stresses and strains that actual hill climbing presents. If you can try to regularly tackle a variety of hill types - from long shallower ones to really steep. Cornwall and Devon are characterised by short steep climbs with nasty descents (I actually crashed on a descent on day one last year and cracked my elbow - not bad enough to stop me riding but needed fixing once RAB was over to avoid any long term issues). You then get longer more sustained climbs until the start of day 8, where you get the nasty climb of The Lecht just to mix things up a bit.
Also, as you get closer further into the summer make sure you do as many back to back rides as you can - such as a Friday hard turbo session, a faster paced Saturday ride for 3-4 hours, a long slow ride on the sunday (5+ hours) and then a turbo session on the Monday. This will make coping with the day after day centuries (over very challenging courses) much easier. I would also say make sure you get a few rides in that are well over 100 miles - some of the days are 120 miles and hilly.
In terms of training plans, I used the low volume sweet spot base 1 and 2 followed by low volume sustained power build. These were basically to ensure that I had some training consistency around any outdoor riding that I was able to fit in. .
Where are you based? I’m central London so my riding included at least one evening a week doing hill repeats around the short sharp hills in Highgate, then surrey hills at the weekends, with turbo sessions on top. Basically, the more you can do and the more specific it is to the challenges of the ride the stronger you will be when you get to the start line and therefore the more you will be able to enjoy the whole process. I was pretty strong in 2013, but found it all really hard. I was a lot stronger coming into it last year, and therefore even though it was hard I found I could really enjoy the whole experience.