I use a Leatt Coolit phase change vest. I keep it in a waterproof Ocean Pack 2l, which I fill with enough water for it to absorb. Or I soak it in a sink/bath etc roll it up and put it in bag. It feels awful putting it on soaking wet, but once on and moving, its bloody marvelous.
As recommanded by @Mac of this parish. Had a look at them but kitting the GF out for this trip is getting expensive and another two of those is going push the budget a bit too far. Might check out one of the cheaper wet options, such as the one suggested by @bootsam, and see how well that works. Jumping into a stream or the sea in my kit maybe an option, have ridden with friends that have done both... Yep, I can see how it would encourage mail order which is maybe their preference . I do like the quality of service that they offer by doing this but on the other hand the long journey plus a wait can be inconvenient in terms of scheduling. Just as well that there wasn't a queue when we went as GF needed to get back to Cheltenham for a work appointment and we were chasing the clock on the sat nav all the way back, made it just in time. Bought her a Klim Marrakesh which she's very happy with.
Each to their own, inclusivity and all that, but I've always found a little sister to be far more interesting than a big brother, but not too young obviously, modern inclusivity doesn't go that far in my book. Anyway, surely an 851 is an older sister, having been born earlier.
As Nasher says, "big brother" - in what way? whole can of worms here, if meant as in engine cc, well an 851 SP2 was 888? what's a few cc between family anyway..