I don't think anyone should own a WR camera without at least one WR lens (especially in the UK
) .
The 18-135 is a good choice is this budget range, it is reasonably fast to focus and almost silent (which is a great feature for not scaring birds and disturbing people).
There have been almost no reliability issues with this lens. Also, it is reasonably sharp, but not like a prime. If you want to really see how sharp the k5iis is, get an old M or A series 28,35 or 50mm lens, there should be plenty around at bargain prices.
The camera body itself may be (IMHO), the best Pentax has ever made. There are times I think it actually out does my K-3.
The Nikon is also a great camera, but has no built in image stabilization, That is a big down side. The k5iis can do composition adjustment using the IS and astroimaging with the O-GPS1 add on module.
Another big downside are also the limitation on what old Nikon lenses that will work. Basically almost all pre-1977 lenses won't function unless you modify them, The Pentax can use any Pentax lenses ever made as well as any other M42 with a little adapter. So you basically end up with a massive selection of great lenses at bargain prices if you go with the Pentax. That was the basic reason I switched from Canon. All my beautiful FD lenses became useless when they came out with the EF mount.
The final big downside is the lack of focus confirmation on manual lenses. You won't need to chip an adapter or lens to take advantage of this feature. Simply put the old lens on, focus until you hear the beep or see the box light up in the viewfinder and you're done.
The advantage of the Nikon is the speed and accuracy of their auto focus system. This is where Pentax is way behind the times. If you take pictures of sports, the Nikon is excellent. The K5iis is acceptable, but you have to use continuous AF and burst mode to get keepers. Technique can overcome some of the disadvantage.
I hope this helps give you more info to make the best informed decision