VAT or not to VAT

I'm starting work tomorrow on a large mural for a school in Southall in London. A curious thing occurred, they contacted me just a few days ago with concern that I wasn't registered for VAT.

I explained that although I do pay tax and am registered with the Tax man I earn below the annual threshold (currently £68,000) and choose not to charge VAT. I thought that customers would be happier if I didn't have to add an extra 15% onto the bill but I was wondering if others below the threshold find it beneficial to charge VAT.

I suppose it does give a more professional image when you charge VAT.