The default assumption here that someone has £30,000 per year to put into savings seems a little bit high for most UK salaries?
There's also been a cultural shift around FIRE in the last 10 or so years. The original mindset was focused on pretty extreme thriftiness and a general DIY approach to lowering annual expenses to maximize saving rate. Nowadays, the focus is much more on getting the biggest number possible, and maintaining a fairly affluent and luxurious (by early FIRE standards) lifestyle into retirement. To some extent that shift was fed by the money-printer near zero bound interest rate boom years of '08 to '19, so it will be interesting to see whether it maintains that trajectory.
I know a few people on ~£140k which is definitely high, but getting much past that is harder. Most mid-to-senior developers I know (in and around London) are on £65k to £120k.
For a regular SWE, $200k (~£160k) is ludicrous for the UK market, but please feel free to offer me a job for that or point me toward the job postings that offer that.
Going contract may be better, but keep in mind you spend time between jobs and chasing leads, and you're unlikely to get a stable years-long contract, especially post IR35 when HMRC will determine you to be a hidden employee (and rightfully so).