Is it possible that they are just not showing the VAT element, the price may be VAT inclusive? I know the EU forced the Government to include VAT on tolls as they are a beneficiary of VAT charged. Can't imagine they would just drop that demand now.
as per Yaffle above, the westlink is vat free as its government owned - that's why prices reduced for commercial vehicles last year (they could claim it back previously) but there was no reduction for private cars.