It doesn't actually matter what you declare or don't in the UK .
The ATCO or other service can and will allocate an emergency response which they deem appropriate.
As soon as you mention you have a problem truth be told the whole shooting match is out of your hands be you declare pan or mayday or even don't declare at all and just mention that you have an issue.
The only way you can ensure you don't get met by trumpton on landing is by not saying a single thing if you do have a problem.
I would suggest though that if you have any feeling at all that you are uncomfortable just tell who ever your speaking to and it doesn't matter if its pan mayday or "I am unsure where I am"
Thankfully in the UK they really don't mind you having a problem. What they do mind about is if you don't say anything especially if its a navigation cockup and you start infringing controlled airspace.