It's very, very situational (each situation has unique circumstances that affect the end result) and hard to give a meaningful, actionable answer to.
But a key factor is going to be timing. Any structure you set up needs to be set up in well enough in advance that it can be reasonably considered set up in good faith and not for the purpose of avoiding tax. I.e., if you're about to pass away in the next one or two years (hopefully not!), then it might be too late to set up something offshore that would hold up with the HMRC takes your beneficiaries to court. The courts would in such a case likely to deem it a fraudulent conveyance and declare the structure null and void for any tax saving purposes.
Speak with fiduciaries and trustees in jurisdictions popular with these types of arrangements (Isle of Man, Jersey, Guernsey, Bermuda, Caymans, BVI, Cook Islands) and explain your situation. Start there, but they might guide you towards Nevis or Samoa or other asset protection type jurisdictions. These service providers will help you find solutions.
Additionally, speak with a local attorney who understands UK law. Make sure that what you're doing is actually legal. There might also be less exotic ways to if not avoid then at least reduce inheritance tax.