Following on from another thread.. Ford Ka is dated, built down to a price and not that modern. But it is said to be great fun to drive and having spent some time on the motorway in one it seemed alright. £4300 would get you one with change. Dated on safety, only 3 stars.
Looking at your list:
VW Polo - Reliability issues, pay a lot for the badge. Older models within budget.
Nissan Micra - Within you budget, insanely easy to drive and very comfortable, if lacking in excitement. Within your budget, common driving school car. Electrical gremlins but otherwise ok.
Peugeot 207 - Too new, don't think you could get a decent one for £4300. Unknown quantity but said to be better than the 206. If you meant 206 you could get a decent one for that sum but it will depreciate like nothing else now a new one is out.
Vauxhall Corsa - New one outside your budget, old version nowhere near as good but comfortable, common and therefore cheap to service/run. Easy to drive as number of driving school Corsas show. A tad overpriced and stingy on extras.
Renault Clio - Renault make me nervous on the reliability. Want to watch the bonnet catch issue, safe cars though and supposed to be entertaining. New models outside budget.
Toyota Yaris - See how you find the driving setup/central speedo. As long as they're serviced they will not break. Though when they do it might cost you a fair bit. New outside budget.
Ford Ka - see above. Replacement due.
Vauxhall Agila - Cheap but not a lot else positive to say for it. Definite no no for motorway miles. Replacement due. Basically a re-badged Suzuki Wagon R.
Hyundai Getz - Hyundai = owner of Kia. Korean company known for long warranties and reliable if cheaply made cars. Interiors can be pretty naff, residual values not great, mechanicals reliable. Replacement due.
Other cars to consider:
Ford Fiesta - similar to the old Corsa, but supposed to be more entertaining to drive.
Citroen C2 - Funky styling, may fall apart. Bit small.
Honda Jazz - repeatedly wins awards, higher mileage older models within price range. Holds value well. 1.4 might be useful.
Skoda Fabia - more grown up car, Volkswagen mechanicals for a cheaper price. Combination of Golf and Polo bits. 1.4 and 1.2 should be alright on the motorway.
Seat Ibiza - Volkswagen Polo dressed as a Seat. Same issues, very similar car. May be less expensive.