Okay it seems to me the op has 2 choices.
1 - Go with a solution that generates html pages such as Actinic.
2 - Go with a solution that uses a database which generates the pages on the fly.
As Dave (quikshop) said any of the above solutions will give a great looking site with the right designer.
I don't think an open source
free solution is the way to go at all. The op seems to doubt their skills and they need a solution with good support. So paying for a package of some form has got to be the way to go.
The arguement of whether a designer will be in business x number of years/months down the line doesn't really exist as long as you have the code for your site. Hosting is much more important. If a designer stops trading and you need your site fixing or changing, whether it uses a database system like oscommerce, vpasp or a package such as Actinic then getting someone to fix the problem or update the design would be easy. Just search for Actinic designers or OSCommerce designers in G and you will see what I mean.
Likewise the fact that a software developer is UK based is immaterial as well. Our software is developed in Austrailia and they have support teams in Austrailia and USA, so support is excellent. There are also numerous developers around the world who I could contact if needed. In the modern world distance is immaterial, so picking software simply because it is from the UK is nonsensical.
It then remains what type of package you want to use and this really depends on how easily you want to be able to update it. I have used Actinic and loads of similar packages and they can be difficult to update, especially if you are going to have a product in a number of categories. However with a database backend updating can be very easy.
My recommendation would be to go for a package which uses a database backend and for which you have to pay for. This will give you decent support (not just a good community) where you can give your access details to an expert and they will solve the issue. Yes you may need to pay for it, but at least you know it will be fixed properly. We use VPASP (
www.vpasp.com) which I am more than happy with, but there are loads of alternatives.