This from a seasoned traveler….who stays within the limit…
Buying wine duty free
The duty-free allowance per person is
18 litres of wine,
16 litres of beer, and
4 litres of spirits (or 9 litres of other alcoholic drinks like port and Champagne).
Up to these levels there is no expectation to declare that you are bringing anything back and two people in a car can save over £150 just on the wines.
If you bring back more than this amount you are expected to complete an on-line declaration which details litres of wine you’re bringing back and the price you paid. You can find the form via the Eurotunnel site or directly via the uk.gov website. You will then have to pay on-line an amount based on excise duty of £2.67 (incl VAT on the duty!), plus 20% VAT on the wine purchase price in France.
This will add over £3 per bottle to the total cost, and possibly much more if you are buying expensive wines.
With this form you simply drive through the Nothing-to-Declare section on returning into the UK.
But, if you want to bring back more than 90 litres, be warned! If you try to enter more than 90 litres on the on-line form, you are told that this will be treated as a commercial transaction and that you must report to customs with your purchase on arrival.
We didn’t want to spend hours being grilled at the terminal so didn’t test this option.