Bridesmaids/best man and destination wedding

If your partner wants it to be just the two of you, and the whole point of eloping is to avoid saying vows in front of anyone you know, then it seems counter-productive to invite a bridesmaid and best man to tag along.

As to whether you can give them these roles for your celebration in the UK, it really depends on what type of celebration you are having. If you are making a big entrance, having speeches or any of the other formal weddingy things, then you should be able to find a role for them, but if it's just a meal and looking through your photos. The best thing to do is to work out what kind of celebration you want and what kind of role you envisage your friends having in that, and then ask them if they would be interested.

As for registry, perhaps have one but don't publicise it unless people ask?

If you are happy for speeches, you could always ask them to do one each. And maybe put them in charge of organising the photos/video for people to look at? And, as you say, involving them with hen & stag events and planning, if that is what they would like. The beauty of modern weddings is that, legal stuff aside, you can pretty much do what you want, so you can create the role for them that you want them to have.