Steps to your registration
If you spend any money advertising your web address and either you
or the printer or the person reading your advert, type in co.uk rather
than .com [or vice versa] some other business could be benefiting from
your investment.
If you intend your marketing to be international, as most website's
are, .com ensures that your domain will be recognised world wide, and
.co.uk ensures that UK based searches will be more effective.
The costs are only for the name which is registered for two years,
you do not require two websites, one will lead to the other. See for
example www.asplash.com and www.asplash.co.uk [which will re-direct
to the .com]