You'll need to get the lights changed for RHD units, the speedo changed or converted to mph, and to ensure that all the light lenses are the correct BS standard colour.
Assuming it's a current or recent production model, this shouldn't be an issue. However, for cars not generally sold here, or for kits, or very old vehicles, you may need SVA (Single Vehicle Approval), which is like a very stringent MOT (although you only take it once). Vehicle with Type Approval bypass this.
Read this from Customs. If it's not a new car (as defined by being more than 6 months old AND having travelled under it's own power for more than 6,000km), then there's no VAT to pay (read section 10), otherwise there is.