Doesn't help with the comfort factor, which could easily be a worthwhile reason to change.
But if you're looking to lose your current, proven reliable vehicle for an economy saving in the form of fuel savings.
18k miles per year isn't insignificant, nor is it all that high.
Going for the up to £28k purchase, I assume would be mostly financed on HP or PCP. Lease of course is a cost for a service, but you're left with no value at the end.
I'm a bit tired, so forgive me if my maths aren't perfect, but consider these figures as the economy saving may not be what you had hoped when the additional lease / finance / higher depreciation rate for a higher value car are taken into account.
Let's start with your Civic... worth £6k now, 2 years and 36k miles down the line, let's guesstimate it'll be worth £3k at the end of it.
36k miles at 34mpg at current rates of approx £6 per gallon of petrol, (36,000/34)*£6 ~= £6,350
So total cost to you (not including servicing and consumables)... approx £9,350
Let's look at your lease choice, assuming you use all your budget to give you the most comfortable car available.
Usually you put down 3, 6 or 9 months for your initial rental.
So you'd have (6+23)*£320 = £9,280
36k miles at 50mpg at £6.50 per gallon of diesel, (36,000/50)*£6.50 ~= £4680
So total cost to you... approx £14k
and you can see the difference in fuel saving by itself over 2 years is just a shade under £70/month.
If you were to purchase, even on a good interest rate & were smart in picking a decent car at a reasonable point in it's depreciation curve and one where the value doesn't fall off a cliff with miles... you'd be hard pressed to improve upon the leasing cost.
The additional £2.5k/year cost to you for a more fuel economical vehicle, doesn't sound like the saving you were after.
Your best option to improve your overall economy & get into a more comfy seat for the longer trips... would be to chop in the Civic and find something diesel and reliable around a similar price or maybe only a little bit more... rather than going for a much more expensive vehicle or a lease.
The only logic to getting a lease or purchase something of higher value would be if you simply fancy a newer car.