After spending faaaar too much time I've finally got most my UK addons to play ball with P3D. Heres a few install notes as a heads up to anyone that has these addons:
Carenado Seneca II - Main Installer found P3D by itself and installed without a hitch. Patch installers defaulted to a non exsistant FSX directory.
REX - Installed to its own Directory. Requires a one off $5 fee to unlock P3D Compatability. Otherwise worked without a hitch.
UK2000 Xtreme airports - Installers found the P3d main directory themselves but didn't pick up on senery.cfg directory. They can be pointed towards the scenery.cfg through the advanced setup or just ignored and manually added to the scenery library after the install. Note, a couple of the installers crash if you select 'no' to the add scenery dialogue. Just click 'yes' regardless of how you're installing.
UK2000 VFR Airfields - Installers found the P3d main directory themselves but didn't pick up on senery.cfg directory. They cannot be changed like the Xtreme installers, you have to manually add to the scenery library. Note - If you activate certain airfields from Vol1 (both version 1 and 2), they will corrupt autogen tree textures. I gave up trying to pinpoint the exact culprits but I think I narrowed it down to 4-5 of them. Ended up disabling Vol1 out of frustration. Edit - seems to be a P3D lack of VRAM issue:
http://www.uk2000scenery.org/forum/index.php?topic=7604.msg52565#new
Aerosoft VFR London X - Wouldn't install without seeing an FSX.exe (doesnt matter what is actually is as long as its named FSX). I would reccomend putting the exe in its own directory - putting it in the P3D directory caused it to install into the GenX folders for some strange reason...
Horizon GenX - Installed to its own directory. Didn't pick up on scenery.cfg directory. V3 patch installers complained about lack of FSX but installed regardless. Has to be manually added to scenery library. All 45 enteries (yay...). Given that Horizon have announced they arent supporting P3D I'll upload a bat file I use for automatically installing freeware later.
Earth Simulations Treescapes - Installed into GenX directory without a hitch.
FSX Power Project - Doesn't come with installer. Manual install worked without a hitch.
UK ANO/Mills/Refineries - Doesn't come with installer. Manual install worked without a hitch.
I also made a 15 min video of a hop from London City to Heathrow. Whilst it isn't buttery 60FPS smooth I am surprised to see it hold fairly steady at 15FPS and even get into the 20s on approach to Heathrow. It does struggle a bit with blurry photoscenery in places so I might try and move it to my SSD and try again at some point. In FSX I would be lucky to get above 5FPS anywhere along that route. However I want to try a similar thing with the Airbus X once Aerosoft fix it - the Seneca was the most complex aircraft I had that worked without too much hassle. Also I still havent got round to fiddling with the AI yet which will probably make an impact:
Note - the video is more to demonstrate the stable FPS more than all the pretty sights, hence some fairly rapid and jerky panning. Also I'm still battling FSUPIC to get the controls and axis set up properly, hence the borderline hyperactive Y-axis! Final excuse - My internets rubbish so theres a hefty bit of compression in there as well...