£14k is a good healthy budget which gives you plenty of options. However assuming your commute is 16 miles each way then you're not really doing enough mileage to warrant a diesel. they cost more to buy, are more complex and thus more likely to go wrong and over all you probably won't save any money buying one unless you do quite a lot of distance mileage personally on top of this. Despite the fact that you don't keep your cars that long I'd be wary of another Focus. They're lovely to drive but the 1.0 Ecoboost is developing a nasty reputation for problems, later cars were reengineered to solve these issues but I have doubts about its longterm durability and having driven several I've found them lacking in torque especially in the heavier Focus. The 1.6 PSA diesel is notorious for problems and should be avoided and the newer 1.5 diesel which is based on it is still an unknown quantity. The 1.5 petrol Ecoboost with 148bhp sounds like a good bet though and is likely to robust if looked after. I've yet to drive one to see if it matches its claimed 50mpg economy but if it comes close it'll be a brilliant all rounder. The Megane is great value used and comfortable. The 1.2 TCe engines seem to be robust and the tried and tested 1.5dci is quite capable of more than 150k if looked after but they crash in value used and thus likely to cost you more to run over all especially the petrol cars which are hard work to sell on. The Civic is a good bet. Not as much fun to drive as the Focus but the 1.8 VTEC petrol is almost unburstible and they're well made, decent on fuel, practical and hold their money better than rivals. The 1.6 DTEC is a fantastic diesel option (we have it in our CRV) which to date isn't causing any DPF issues I've heard of, provides strong performance and offers genuine 70mpg capability in the Civic, we get over 60mpg in our CRV driven lightly over distance and around 56mpg day to day. Look out for clutch judder on the diesels though. The latest Vauxhall Astra is pretty good value right now and I'm hearing good things about the 1.0 turbo. The 1.6 diesel is based on a Fiat design though Vauxhall dealers are denying this but it came from the deal between the two firms before it ended. It's as yet unproven and the previous Fiat engines models haven't exactly covered themselves in glory regarding reliability. Another good option for you would be the Mazda 3 2.0 petrol. It's an excellent car to drive, is capable of 50mpg if driven well and has a bombproof normally asperated large capacity engine instead of rivals small highly stressed turbocharged units. It lacks a little midrange pull but it's great fun to drive and is unlikely to ever go wrong. The Toyota Auris is another worthy too. The latest 1.2 turbo petrol is good on fuel and quicker than the older 1.6 and although a bit dull to drive it has a 5yr warranty and a great reputation for reliability. The 1.4 diesel is a Toyota unit and likely to be robust though DPF issues aren't unheard of but the 1.6 diesel is actually a BMW unit and I don't have confidence in BMW diesels for longterm reliability, they almost always become money pits, though this is at an age well beyond your likely ownership.

I'm not endorsing the Megane, but on HJ's 'Cars for Sale/New section you can actually buy a brand new one with the 1.2TCe engine for around the OP's top end of the budget, or its sister car, the (boring) Nissan Pulsar, but with an even higher discount (up to 26% off the list price - both actually considerably more expesive than was the case a few months ago, where both had discounts of well over 30%). You have to wonder sometimes whether a) car firms are making huge margins on cars if they can knock off 25%+ and still make money, and/or b) they've seriously compromised on design engineering quality to be able to sell them at the lower price and make money, compared to cars attracting less of a discount despite selling in similar numbers.

Andyway, I'd wonder if its worth the OP keeping their current car, unless its been unreliable - if it is going fine, then its in that 'sweet spot' period where running costs are still quite low and depreciation is minimal. I suppose it depends on what mileage its done and the quality of the roads its been driven on (e.g. lots of potholes, speed humps locally or not - my parents cars (Ford Fiesta) suffered from many suspension-related issues around this time due to being driven over such roads (before getting on motorways, etc) despite having a low mileage. Might be worth them waiting until that first big bill comes in for wear-and-tear repairs (e.g. suspension parts) before changing - the 1.6 petrol (similar to that in my Mazda3 mk1) is perfectly fine for the OP's journey.

I wouldnt buy a Megane unless it has improved build quality from the old model. We have some of the old Meganes in our work fleet and they are falling apart. They also are dull to drive.