You can start up your own limited company for £15:
https://www.gov.uk/register-a-company-online. There are
online sites that will do it for you for an extra £20/30 and that saves having to write out your own Articles of Association, etc, there's further info on all that here if you wanted to tackle it yourself:
https://www.gov.uk/limited-company-formation/register-your-companyAll you need to bear in mind is you might need to mark someone else down as second company director/secretary, they may have changed the rules on this recently. If you a have an accountant (which is worth the cost for the savings in tax they'll get you/headaches filling out returns and sorting out Employee payslips, etc) they can usually mark themselves down for this and can provide a registered office address. A good accountant will probably cost you under £1k a year in fees (Corporation Tax return being the biggest cost).
You could set up a nice brochure website easily using Wordpress, it's not hard as you're not selling online, you just need a contact form page and some pages for home / commercial / home cinema or whatever you're specialising in. If you wanted to pay for this I wouldn't look at spending more than £500 and really that should include a nice logo, anyone offering you a billy basic site for your business for more than that is fleecing you. Submit yourself to some web directories like checkatrade and you won't need to spend that much on advertising, I don't know much about your industry but I'm guessing a lot of people still flick through the Yellow Pages, but since even they're online now...