Assuming you’ve already used one of the comparison websites to get a range of prices, you could try some of the insurers who don’t feature on these sites - e.g. Direct Line, Aviva (I think Aviva may now be on some comparison sites though). Don’t necessarily go with the cheapest price - an insurer with a top notch claims service but who’s not the cheapest is IMHO a better bet than an insurer with the cheapest premium and a rubbish claims service.
You could also do a Google search on ‘insurers of performance cars’, which will probably give you the names of some specialist insurance brokers who have agreed a special scheme arrangement / discounted rates with some insurance companies. Additionally, you could try the traditional route of using a high street broker who can search the market for you - they may also have preferential rates / premiums with some insurance companies, so may be surprisingly competitive compared to some of the big names on comparison sites. Specialists are also likely to have more knowledgeable staff if your car is modified / non standard, so they’ll have a better understanding of the insurance risks associated with modified / non standard cars than some of the large insurance companies.
I seem to remember that Sky Insurance used to get quite a few mentions on this forum - they may have been a forum sponsor then. They still exist but are now Safely Insured, so you could always give them a try too. They specialise is performance cars, modified cars and young drivers.
https://safelyinsured.co.uk/blog/2021/03/sky-insurance-is-now-safely-insured/Be aware that most insurance companies increase their prices the closer you get to your renewal date, so drivers who leave sorting out their insurance until the last minute will usually be quoted a higher premium than someone who gets things sorted a few weeks in advance.
