If you want to participate in a party as a visitor, this is absolutely allowed. Even if you call it a "wedding ceremony" – it really has no legal meaning. But, you can not enter into a legal relationship with a visitor visa. You can not get married, you can not register a civil partnership, none of that.
If you are asked at the border, honestly answer that you intend to celebrate your to come up wedding in a foreign country with your fiance's friends and family in the UK, then go with your fiancé to get married elsewhere. Insist that you do not intend to get married in the UK and do not intend to live in the UK.