Edit - just found this on the Met Police website:

"How can I take my firearm or shot gun abroad?
You will require a European Firearms Pass, which can be obtained by sending an email or letter request to the relevant enquiry team listing all the weapons you wish to take abroad with you."

Yes, I'm aware of these kinds of permits to allow a shooter to travel (to attend competitions or hunt in different countries), but I'm pretty sure that it wouldn't allow concealed personal carry of a prohibited type (handgun) in the UK, and certainly not for self defence!

Edit: from the Met website again:
"Can a visitor to the UK possess shot guns or firearms?
Visitor to the UK may be granted a visitors permit, which allows them to have in their possession firearms, shotguns or ammunition. Please see our overseas visitors section.

Under section 17 of the 1988 Act a visitor to Great Britain may, if he is granted a visitor's permit, have in his possession firearms, shotguns or ammunition without holding a certificate.

The holder of a visitor's firearms permit may have in his possession any firearm (but not purchase one), and purchase, acquire or have in his possession any ammunition, to which section 1 of the 1968 Act applies."

I believe the Home Office ones I mentioned above are very 'special case' ones, which would be reserved for VIPs, vulnerable ex-forces targets etc.