Having the .com will only be useful if you're targeting international customer. Your your business is targeting the UK stick with your current domain name. It's perfect for your needs and there's no need to fix what isn't broken.
If you do want to switch to the .com do a 301 redirect from the .co.uk to the .com. It's simple to do and a must for SEO and usability purposes. Put this in an htaccess file in the root web directory of your .co.uk site:
RewriteRule (.*) http://www.yourdomain.com/$1 [R=301,L]:
This will tell users and search engines your site has moved and redirect them to the same page on the new website. For SEO it will also transfer PageRank to the new URLs (with a small loss due to a dampening factor but it's minimal).
Also tell Google directly that your changing domains.
Just keep in mind during the transition you will see your rankings fluctuate and also see lots of 404 errors in your Google Webmaster Tools. This is normal.