Welcome to Vodafone Community
There used to be ways of diverting a call to get a UK ringtone but that was messy and it may not even work now.
So short answer is you can't personally alter how a person hears the dial tone.
You could always divert straight to voicemail and then return the call if your not wanting a person to know your not in the UK.
Just a thought.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.