Well according to there website, some of there customers are using Fiat 500’s so you may get away with a 1000cc model which is probably road tax exempt.
Since 2017 road tax (VED) has been a flat rate, currently £155 for most vehicles.
Prior to this it was based on CO2 emissions so it is likely to be a smaller capacity petrol or diesel you are looking for, you then need to check towing capability.
Buying a 5+ year old vehicle may however prove a false economy to save £155 max on VED.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.