Hi and Welcome to Web Hosting UK forums!
If you are planning for hosting an ecommerce site, it is always necessary to consider a hosting provider that can offer you 100% Uptime guarantee. Now, it depends on your hosting requirements, how big is your website, how much bandwidth do you need etc.
On shared hosting plans you are offered 99.97% Uptime at max. No one can offer you 100% Uptime Guarantee on shared hosting plans. It is only possible to offer 100% Uptime Guarantee, if you consider Dedicated Server Mirroring Solution.
Please let us know your requirements in detail, I am sure either we, as a forummate or any Web Hosting UK member will surely help you in suggesting the best suitable plan.
About support, WHUK offers 24/7 reliable support through Live Chat, Phone Support and Help Desk. You are never left alone when you are in need for help. This really makes WHUK a genuine, reliable, supportive and the best hosting provider, I have seen ever.