If your budget is up to £6k, try to get a 30kWh (28kWh usable) Leaf from 2015-2017 with 12/12 bars of battery health. This will give a good 70 mile range. I would avoid the 24kWh Leaf now as they have low range (maybe less than 60 miles), unless you need a bargain and don't have to travel far. Avoid 2011-12 as battery capacity should be too low now to be useful, even for short commutes.
The 40kWh (36kWh usable) battery version has better range (up to 140 miles) and has more tech on-board, so worth the extra. You can get one for around £9k now with decent mileage and spec.
N-Connecta is good level of spec on the 2018+ models; Tekna is top spec and available across all versions.
62kWh (56kWh usable) models are more expensive, starting at around £12k used. At that price, there are probably better options now unless you need something that the Leaf offers in terms of tech or size of car?