Hello,
If someone was convicted of a sexual offence in the country of which they are a national of (not the UK) when they were 18, could that stop them from visiting the UK? Even if it happened more than 5 years ago? 
Are there any waivers available if that person was coming to the UK to visit their partner or if they were travelling with their spouse who is a UK national? 
Any help and advice would be much appreciated.
Thank you

Depends on what country you are coming from.
How long you where convicted for.
What you where convicted for.
Are you coming to visit or for work.
Do you need a visa or visa free.
And finally if you are going to fill in the forms honestly.

With out knowing the above nobody can say a definite yes or no.

That is a fair point, my apologies 
Country coming from - New Zealand
Length of conviction - 9 months remand and 5 months ankle bracelet awaiting trial then 2 years house arrest 
Convicted offence - digital rape 
Reason for visiting UK - visit long term partner who is a UK national 
Visa or visa-free - presumably visa since there is a conviction 
Fill in forms honestly - yes