In the UK, funeral plans are regulated by the FCA. This means that as long as you choose a funeral plan provider who is authorised by the FCA, your money is protected. If the company who you bought your plan from goes out of business, your plan will be transferred to another funeral plan provider. You have the option not to transfer and to ask for a refund instead. Though keep in mind that the refund might not be for the full amount you originally paid.
You can use the FCA register to find authorised funeral plan providers.