Many thanks. There should be comparatively little problem in your obtaining a notary or solicitors certification to your degree certificate itself certifying it as a true copy of the original but this is unlikely to help you if your employer requires that it is legalised because in order for the certificate to be legalised it must be issued by what is known as a "recognised educational body". You can find a list of such bodies here:
Certainly Regent's University appears on the list but the instutation you studied at does not. What is fairly common is that institutions that are not reconised bodies themselves award degrees though associated recognised bodies - i.e. they have an agreement with a recognised body that they will certify the qualification. It may state on your certificate the recognised body the certificate was issued by though it may not.
In terms of moving forward it may be a good first step to contact the university in the US and ask them to check their records to confirm which body they awarded your degree through when they were in London or if they were at the time a recognised body and ask them for evidence of their then status as such. They have records they can check and come back to upon. If you can obtain this information, if it is not otherwise clear that the certificate was issued by a recognised body you would need to consider retaining a notary public rather than just a notary, who can verify the information you obtain and then certify the degree certificate as being issued through a recognised body which can then be legalised.
If you find that your degree was not issued by a recognised body in the UK the FCO will refuse to legalise it. If the university issued the degree certificate through an equivalent foreign educational recognised body then you would need to look at getting the degree certificate legalised by the foreign country that recognises that body - i.e. if the university only has recognised status in the US even though it had a London base at the time, then you may need to get it legalised in the US instead.
I would be suprised though if the university was not operating through an associate recognised body at the time as if they did not their degrees would be far less attractive to students so hopefully with some investigation with them you can confirm which body that was.
Does the above answer all your questions? If it does, I should be very grateful if you would kindly take a moment to click a rating for my service to you today. Your feedback is important to me. If there is anything else I can help with please reply back to me though