keep the tv you have until a tv comes out with a feature you "need" or "want". upgrading for the sake of it is just throwing money away. in 6 months time another tv will be out which costs less and beats your £1.5K tv, then you will feel like either you need to upgrade again or have just wasted your money.
put it this way i had a budget of £1200 - £1400 for a new tv after my old one stopped working (after 10-15 years of use). i decided to go for a tv which cost me only £769 (PS51D8000). now that is roughly half my budget. why did i do this? i realised that the law of diminishing returns works on everything, as in the more you spend the less you get for your money. now that tv i bought was samsung's 2011 flagship plasma model, the same tv but this years model was £1600 when i purchased mine. although a better tv, was it really £831 better? in my opinion no, it was only worth maybe £300-£400 more than the tv i bought.
imo it best to go for bang for buck rather than the best. because when you buy the best, in 6 months time it is no longer the best as it has been replaced by a newer model. therefore in order to have the best you need to upgrade every year. which imo is a waste of money.
keep the tv you have for another few years, then upgrade when you actually need a new tv. spend the £1500 on a holiday to a far away land you have never been to or invest it.