What type of domain do i need?
If you are predominantly selling to customers in the UK then you should use a .co.uk domain. Customers do consider this when deciding and may feel more comfortable with local country provider.
If your ambitions are greater and you have the budget and opportunity to sell world-wide then a .com domain is definitely the right option. Someone in Australia or USA might not think your business is as relevant to them if they see your website has a .co.uk address.
If you are a UK based start up business, it may prove the more sensible option to launch in the UK only with a .co.uk and review your global ambitions once you have proven the business locally.