Renovations, big or small, will tend to be a little pricey. How inexpensive exactly are we talking about here? The cost usually depends on how much work needs to be done in order to turn the space into what you need it to be. You may want to have the contractor come over to see the space in person before you can get an estimate of how much the job would cost you.
In that regard, I looked up building contractors in the Hastings, Sussex area and found at least one name that might be able to help you. The company’s called Paul Mitchell Plastering & Building Contractors, and they appear to do renovations, refurbishments, installations, and conversions – which, to me, are exactly what you want to do with your spare room. I’m not sure how they do their pricing but by all accounts, they seem to be flexible when it comes to budget. It wouldn’t hurt to ask them, so here’s their website. I hope this helps.