Lastly...135 f2 as my one long lens?

Click this link for my review of the 135L

I'd say ABSOLUTELY go for it!

I've never used it with extenders, but here's my thought on that... if you think it'll rarely get use with an extender, then rarely carrying something larger than an extender shouldn't often be too much of an issue. Conversely, if you're going to use it with an extender ALL the time, then you probably want the very best results possible. To me, extenders are probably best suited for situations where they save THOUSANDS of $$$$ - this is not one of those cases.

So, rather than get a 1.4x extender, why not pick up the EF 200mm f/2.8L II USM instead? I bought mine, used, for about 15-20% more than the price of a 1.4x III, new (these aren't often available used). To me, it's TOTALLY worth it to have this "extra" lens around to tackle my (fast aperture) telephoto needs beyond 135mm, rather than an extender. There are a LOT of barely used copies available, at least here in the US, for much less than the lens retails for new. It may be the same there in the UK.

Image from 6D + 200mm f/2.8L II

The same goes for the 2x III extender and the EF 300mm f/4L IS lens. Granted, the 300 lens is quite a bit more expensive than an extender, even on the used market (double the price of the extender, here in the US, for a used copy) but you also get IS (2 stops, in my experience) and likely much better AF performance and IQ.

Image from 6D + 300mm f/4L IS - when pixel peeping, note this is ISO 800 and not ISO 100.

Good luck in your decision!