companyname.com.uk would give you the best seo advantage, if that was an option. Followed by companyname.com if you were after internatioinal business.
I would choose www.companyname.com for SEO which is considered as best for SEO techniques and Main reason that the extension is .com.
.com is still the best anywhere, but for better optimization at local search engine searching and crawling, use local web hosting
.com is international domain name. if your company want to extend out of UK, i think you should choice yourcompany.com some idea.