Brrr...if you are cold natured like I am...might ride through England...but I'd have to pass & rent a car for Scotland.

. Brian could probably advise on that. It has been my experience in Scotland, (mostly in mid July-early August..average temp is about 60*. That is on the good years...and then top that 60* with a steady rain... and you have your July in Scotland. LOL, granted 60* is about what I like to start the day on with the anticipation that the temp will increase, but that is the high temp generally. If it were February here and I went to 60* weather, well woo-hoo I'm in. But going from 85-90* to 60 is rough on me. :O. The other thing is that the roads are different than what we are used to. If you or your counterparts are somewhat inexperienced, you may look at getting them out on some 'rougher' pavement. I have not ridden a bike in Scotland, but from what I have seen I feel that it may be a bit different which might make the bike handling a bit unusual/uncomfortable at first.
It seems that you may just be looking at some day trips? Or were you looking at renting bikes for an extended period of time? I'd be game for planning the trip, and then looking at the weather to plan the 'riding' days. But that is just my opinion, and I will be the first to admit that I am a fair weather rider. I don't want to be cold & wet when riding. (I spent 19 years being cold and wet when I didn't want to be working an outside job).
We didn't go to Scotland this year, but from what I can tell the weather there has been quite nice. Our first year there was 2013 and the weather was remarkable. In 2 weeks, we only saw a few drops of rain where we were in Lanark. (about 45 minutes south of Glasgow & Edinburgh). The last 4 years there, well, it was typical Scottish weather.

. Oh, did I mention the wind? LOL, I love it there, but I really do not know if I'd own the bike if I lived there.
Here is one suggestion I have; I have an iPhone and I have added the weather for the locations I go to or want to go to, anywhere in the world. When I am curious, I just pull that city up and see what the weather is doing. I also use Weather Underground to monitor sunrise & sunset. (Since most of the time when we travel, we are planning hot air balloon flights). So, in Lanark currently, according to my phone, it is 53* the high temp for Monday is forecast at 64*. Partly sunny in the am, and mostly cloudy in the pm. It also gives the extended guess for the weather.

. I hope this helps with your planning a bit. If nothing else, you will know to acquire good rain gear if you haven't already and pack it for the trip!