I don't think there is any cert that employers will want to see on account of your being from the UK. You just need to look at what would be helpful to any pen tester for getting hired.
I don't know what employers look for in the UK, but in the US the CEH is probably the most widely recognized pen testing cert. It's supposed to be relatively easy to pass and will help you with HR and hiring managers. If you're able to pick up additional certs, the OSCP and the eCPPT are good choices; they both require a hands-on/practical test and pen testers tend to respect them more. The SANS certs (e.g. the GPEN) are also very good to have, but they are expensive to acquire unless your employer is footing the bill. I think live attendance is around $4k or so.
If your skill set is focused on networking (as opposed to say web apps), the CCNA is another good add-on. It's fairly well regarded by HR and by technical folks. The basic CCNA isn't focused on security, but it shows you have the basic networking/infrastructure knowledge that most employers will want you to have alongside your pen testing skills.
I wouldn't recommend the CISSP in your case; at least not now. It's sought after by many employers, but it's not very technical, not specific to pen testing and (since you are from outside the US) it won't be able to help you get one of the many security jobs available with the US government or US defense contractors.
Short version: Get the CEH then try to follow up with the OSCP, eCPPT, CCNA or a SANS cert (your pick).
I know you've been around here for a while so I may be telling you what you already know. Still, I hope I gave yo u something helpful.