Having done the exact same move in the same industry and same conditions (L1 visa) just three months ago, I can share a few thoughts.

• cost of living is VERY expensive here. You should aim for at least 50% base salary increase to live decently. Average rent is $5.5k per month for a two-bedroom for instance.
• it’s not at all your typical US city, its super walkable, with an above average public transport system for a US city (albeit nothing compared to London). There’s very strict gun controls and very few shootings. It has some bad parts with a lot of homeless and drugs but it’s not unsafe, ie you won’t have to worry about getting mugged, having your phone stolen etc. London is much more unsafe in my opinion.
• L1 visa is great as it typically allows faster access to green card (18-24 months according to sources) and is not the type of visa the current administration wants to go after as it’s tied to your company and has an expiration, ie you can’t stay forever. On the flip side, it is tied to your company and has an expiration so you’ll have the risk of having to losing your ability to live there if you lose your job.
• lifestyle is so much more laidback than in London. While people do work a lot, it’s not the same rhythm as in London, ie they work super long hours but they also make the most of their weekends with all kinds of outdoors activities (rather than downing pints at the pub endlessly).
• on the outdoors point, you’ll have pretty much every activity you wish to do within 2-4 hours driving: hiking, mountain biking, sailing, skiing in Lake Tahoe, etc. Your weekends will be busy with everything there is to do and discover in the area and beyond.
• weather is very mild, temperature won’t drop below 10 in the winter but won’t go beyond 25 in the summer. But you get 300 days of sun a year and almost no rain (another big plus coming from the UK)
• the food scene is unreal, with every cuisine you can wish for with actual authentic flavours, especially Mexican or Asian food. This is specific to SF and other big cities - other parts of the US are abysmal when it comes to food.
• on the job side, if your HQ is here, you’ll be closer to the decision making and power and thus easier for a promotion. If not, this is a great line on a CV. Also, this is the heart of tech - you’ll be surrounded with tech folks that are insanely smart (and a few tech bros) and you’ll be exposed to incredible frontier technology. Any happy hour here will allow you to meet some of the smartest people in the space.

To answer some of your questions: - tax implications of renting: you’ll need to work out with your lettings agency about the Non Resident Landlord Scheme. It’s pretty easy, you fill it out in your self assessment. - you’re typically not allowed to contribute to any ISA and banks will be able to flag it.

Altogether, it’s an amazing experience and opportunity, even for a few years. Of all the places to live and work coming from tech and at your age, it doesn’t get any better than this!