Mostly only peasants and white aliens do much walking on the streets of Bangkok. Don't expect this to become a focus for city planners anytime soon. If you enjoy urban walking, move to any city in Western Europe where you will find well-maintained footpaths, a more comfortable climate, and less-cancerous air.

i'm sure it will be basically non-stop on bbc news and sky news.

it says 24, not 20

if you will be working, volunteering, studying, or just travelling, you can get a visa on that basis. you are too young for a retirement visa but if you are rich enough to be in that position at your age, you can probably buy an elite visa with the spare change in your pockets.

interesting holiday plans. what could possibly go wrong.

Just go. The UK doesn't enforce this rule nor most of the other virus control measures, except issuing fines (that aren't going to paid) to people who organize parties to make it seem like they are serious.

Phil Jones more than half of that would be spent before he's even released from quarantine

200,000 baht including insurance, flights, quarantine, and normal day-to-day tourist spends (without alcohol). maybe less if your friend will accommodate and feed you. and more if you plan to stay in a 5-star hotel and eat at nice restaurants every day.

Your understanding of the facts is correct. The quarantine fee also includes room service food and covid testing provided by the hotel's medical partner. Many hotels in Thailand are now offering heavily discounted monthly rates, which should balance against the requirement to do extra research on your accommodation (and the risk that you make a bad choice).

No problem. Most "rules" to control the spread of the virus in the UK are more like suggestions with many exceptions.