SEO companies tend to fall into two categories. The larger companies who have a tele sales department and are out looking for companies who are looking for a wider coverage and smaller more local companies who offer a more local service. There are pros and cons for both. The larger companies can sometimes be a little cheaper and offer a service that may be short on the personal side. The local company may have fewer customers and therefore offer that little bit extra.
The most important thing is the expertise. Can they deliver. Promises are easy. If you're looking for SEO in North Wales and simply looking for your customer to come from North Wales or surrounding areas you might be better contacting a local comapany. To be sure the nationals will probably have already contacted you!!!
If they offer a free chat over the phone why not take them up on it. See what they sound like, you can tell a lot from a phone conversation. Will you be able to see them? Can you speak to or even meet the person who actually does the work on your site? Do they sound as if they're hard sell. You will probably get greater flexibility from a smaller SEO company in North Wales but they may take more of your time, but it may well be time well spent.