There really isn't any magical way to get a placement, you just need to email a whole bunch of people. Find some hospitals that you would be interested in going to and see if you can find the emails of any of their neurosurgeons via their websites or whatever. And just send an email saying a lot of the same stuff you've said here. Specifically your 1st paragraph, I wouldn't bother mentioning the rest. I always like to include something along the lines of "I understand if you are not able to accommodate me. If so I would be grateful for the contact details of any of your colleagues who may be able to help and would not mind being approached", but that's just me/totally optional.
I don't know if there are any specific requirements regarding foreign medical students coming to the UK though, so you should probably research that first and make sure you meet whatever requirements there may be.