I'm not over struck on them, so if I wanted a 19" upgrade, I would probable go for a BBS wheel.
However the way I would go about it would be to run the car with the standard wheels on until the tyres had around 3mm of tread left, then buy a set of new BBS wheels and tyres. Offsetting the cost of the tyres that I would have needed to buy soon anyway, would make them around the same as the £1000 factory upgrade. However when I come to sell the car, I'll either have a set of original Austins with usable tyres, or the BBS's that I could sell on and make a bit more money back.
If you've paid out £1000 for the factory upgrade, you'll never see that money again.