What is the budget? You’re saying the Berlingo is worth £4-5k, are you hoping to do a straight swap, or top this up?
But you want to attract girls? They’ll think you’re gay, unless you have a Cooper S, which will fall way short of your specific MPG.
If you want a Mini, buy one, there are thousands out there, most are Coopers (just bought ms njee20 a Cooper Clubman this week), claimed MPG is 42 IIRC, so there or thereabouts. Cooper D will be better, but unless you’re doing mega miles it’ll probably be offset by costing you more in the first place and the higher servicing costs.
You’ve dismissed every suggestion in the thread, not really given much information, and seem to know what you want already. Starting to see why you’re newly single 😉