What Sonos speakers for outdoors?

There aren't any.

You would have to use a connect amp and third party outdoor speakers.

Make sure you adhere to local electrical regulations, you may need a qualified electrician to install.

Would an electrician be needed to install speaker cables for passive speakers? It is still a task that needs some doing and some care in terminations and shielding these from water, but does not involve the kind of hazard that mains power cable laying would. Well made external speakers are also designed to allow the speaker end terminals to remain free from water, so this is very much a DIY kind of job.

I am not sure of the local regulations, but in the UK as the wires would be connected to an amp that WOULD be connected to to mains... I would want to ensure that there was some sort of Isolation / protection.

A local electrician would be able to advise,

The net is probably a good research resource for this for anyone that needs it; I suspect the amp would be isolation enough.

For instance I know a UK maker of passive speakers that does not want to go into making amps or powered speakers in a hurry because that would significantly increase his regulatory compliance hassles from what he has today as just a passive speaker maker.

Apparently the choice is to use a connect amp and speakers or to have a speaker one can take in and out when needed. If I go the later route, what are the pros and cons of a Sonos 5 vs. two Sonos 1 speakers?

Play 5 has a line in and would require one power connection.

2 Play 1's could spread further out without needing to be louder and relaying the sonos net, but would require 2 power connections

I have struggled with the outdoor speaker solution for quite some time. Last year I finally broke down and purchased an a Connect Amp and two 3rd party outdoor speakers. Works great. So much so that I got another Connect Amp this year. The only problem is that I wanted to put two speakers a bit farther from my home. I've been playing around with various waterproof/vented enclosures that I could use, but am still working on it. Any ideas in that respect would be great.

For more recent relevant information, see: https://en.community.sonos.com/wireless-speakers-228992/outdoor-wirelessac-powered-speakers--6787491/index1.html

The only problem is that I wanted to put two speakers a bit farther from my home. I've been playing around with various waterproof/vented enclosures that I could use, but am still working on it. Any ideas in that respect would be great.
Can't you run speaker cables of adequate core thickness to the location and use one of the many external speakers there?