I would try asking local paints shops if they can recommend any tradespeople. Also perhaps ask the council's conservation/listed property officer. Even some manufacturers/suppliers might know of local tradespeople or reference sites.

Is linseed oil paint microporous? I would look to finish with something similar in nature to what is already on the wood. Linseed oil on its own is mainly used on bare wood so presume a pigmented linseed oil would be similar.
Could you try a sample first and do some experimentation?

I use only linseed paint on woodwork. It's an old house thing. You restore windows 200 years old and I doubt they would have survived that long had the paint process not at least started off with Linseed under the modern vandalism. Life of windows isn't so much of an issue these days so that might not be an issue here.

Linseed is an oil like the oils inside the wood. It moves and breathes like wood does,keeps most of the water out, and lets the wood dry out when water gets in the gaps. It is best applied on bare wood with previous coatings stripped off. It works as a paint because the topsurface oxidises and kind of dries.

It can be a complete pain. Apply only in very thin coats and leave for at least 24 hours in warm weather before another coat. Pignemtation tends to be weak and you are likely to need two or three coats. It goes matt quickly and white is tricky to wipe clean. It's a cottage industry and some paints go mouldy. I've not had a problem with Ingleby Traditional. Paint is smelly and can give you a headache if doing it indoors in a confined space.

Benefits are when you need a repaint there is no cracked paint to remove. Linseed just dulls and dusts and never cracks. Just brush off the dirt and paint over, you can use pure linseed oil rather than a pigmented paint. Also the windows might last 200 years.

That is nonsense. I use linseed paint on cast iron too. It's what the Victorians used (I also use a high zinc primer underneath for anything steel) It doesn't soak in very well but it stays on and doesn't crack or peel. Don't bother with primer on aluminium and it seems to work. Painters don't like it as it takes too long to dry. Actually I don't ike it for the same reason and it's annoying to have to wait 2 days between coats.

It doesn't need an expert to apply. Put it on really thin, get on with something else, then another really thin coat in a couple of days. Finish tends to be smooth but dulls quickly. The advantage is it doesn't crack or peel, keeps your wood breathable and is easy to refinish later. If you want gloss the only option would be the cracky and peely stuff you need to strip off every few years before you can repaint.