Desciple, make sure you check all around your area. Your in the UK, and I know without a doubt that there are some really skilled artists in your area. Portfolios are going to be a major aspect, and for a sleeve, you are going to spend a LOT of time in the chair. Even in our own shop, we have 5 tattooists, and each of us has our own flair that we add to every piece we do. For example, I use a LOT of color, focusing a lot on American Illustrative (pick your style like realism or traditional and shade it out all new school, bright colors, underglow) Your best bet is to go in, with an idea, a few references, and let the artist have fun. Just remember that for good work, you want to pay the extra. Find out the hourly rate, plan what you can do every month, and go with it from there.
Recently I just finished a sleeve based on Alice Coopers schools out for summer. We have a whole bunch of just random things, but it all has to do with the lyrics and a simple background to tie it all in.