Opinel? Not in the UK, Sunshine!
Well not the familiar version at any rate - that rotating collar makes it a lock knife and subject to all the restrictions. I think I remember reading that they were bringing out a UK Legal Carry version, but I dont know if it happened yet.
Have a look at this page for examples of what you might be allowed to have about your person, although any slip joint with blade less than 3" should be ok
http://www.heinnie.com/Knives/-UK-Legal-Carry-Knives/c-1-92-641/
I imagine you would be able to get some of these near to home. If so,
dont try to carry it onto the plane - it
must go in hold baggage. (I know you probably know that, but just in case...)
Our law can catch people out. A few years ago there was a stir when a man was arrested in London when a scanner picked up a weapon in his baggage.
He was a senior army officer serving as a military attache to one of our embassies. The weapon was a 2" paper knife included in one of those credit card toolkits. Two inches overall - but it was a fixed blade...
A good reason for carrying a knife could be to open plastic food packaging. As for self defence - Dont. Not with a knife. It would be an issue. Keep your cards and valuables somewhere safe and only carry petty cash on the street. If mugged, hand it over. It'll be cheaper than paying for lawyers.
Having said that, Its not as bad as it might appear. Theres a good chance you wont be bothered, especially if you use a bit of common sense about where you go
Enjoy your visit!