if you were going for the ad500x... it may be worth saving up for an ad900x or ad1000x as they are improved versions.
if you were liking the hd598 then there is the hd600 and hd650. they are a bit "slow" and more "mellow" than the hd598 which makes them easy listening but perhaps not ideal for all people.
as far as beyers are concerned, going premium is worthwhile only if you get 600ohm cans which do sound better than the 250 version if you have the cash and a decent amplifier.
generally this can be said: either people tend to like the dt990 or dislike it. those same people who dislike the dt990 tend to like the hd600/hd650 while those who like the dt990 tend to dislike the hd600/hd650. this is due to them being different in sound signature. the dt990 is more bright, energetic, and acute on audio while the hd600/hd650 are more mellow. the 990 for some can be fatiguing to listen to for long hours although others like it. the hd600/hd650 is easy listening for long periods of time however some might find them more boring or too slow to fit their preferences.
given how uk prices are high however, it would be a significant jump up from the level i discussed before which makes it questionable unless you wanted to invest significantly more into them. the $100-250 market (which includes the hd598, dt990, ad700x and similar) is the sweet spot between sound performance and value. the more expensive you go up the line the less performance gets added per additional cost.
something worth saving up for... perhaps
http
/www.amazon.com/STAX-SR-009-Electrostatic-Earspeakers-Japan/dp/B004W1S0BY/ref=sr_1_1?ie=UTF8&qid=1437567075&sr=8-1&keywords=stax+headphone (i joke, although they are really good headphones)