If you're looking for something to last through grades, compare the CLP220, 230 and 240. There are differences between these models but the higher the code, the better quality the piano in relation to touch, keyboard repsonse and sound quality. Downside is that the CLP240 will push your budget to 1100 GBP at best!
Other models worth checking, again in Yamaha, would be YDP160 if it is out and about yet - will be cheaper than the CLP models mentioned but again sacrificing better keyboard action from CLP230 upwards. See if you can try the YDP-S30 from Yamaha too.
I'm trying to give you a variety of dgitals to try in a wide ranging budget. Personally, if you can afford one, the CLP240 is good value and will give you the hammer action keyboard, better quaility piano sound starts with this model as well as more powerful speakers. This would be a longterm purchase if you choose this model; I think you'll get a lot out of it. But remember to try the instruments before making a decision.
The CLP270 has the same basic features as the CLP240 but with more voices plus 480 XG voice samples. It also has higher polyphony (128) than the CLP240 (64) and a more powerful set of speakers. It also has a larger recording capacity for recording your performances.
I think you really need to have a look at the model for yourself. I can't really see a huge improvement voice and keyboard response-wise as you compare the models, but if you feel those extra voices and things I mentioned are worth the extra 500 GBP or so, then go for it. Personally, for early learning level and onwards, the CLP240 would probably be enough for what you need.
Try for yourself and fiddle around with the settings and see for yourself. I recently helped a lady from our church get her first digital piano. She was a complete starter but like yourself, wanted something that was a high enough standard to last for some years. I helped her track down a CLP240.
Again, this is your call - you're the one that is going to sign the cheque at the end of the day!
As with all digital equipment (computer, consoles, hi-fis, DVD players, etc.) digital pianos are not great investments after about ten years or so either due to the fast upgrade rate. You can be sure that Yamaha will have something else even better than the CLP range in another 5 years or so if not sooner! If they're looked after they will last you well which is why I'm suggesting a decent quality digital with a good sound and keyboard touch that won't be easily outdated or outdone by upgraded models. If you are seriously thinking about the instrument lasting you for grade exams, particularly Grade 5+, an acoustic may be a better investment. Having said that, a crappy, secondhand acoustic will be a bigger waste of money if you are not careful about what you choose. A good bargain on a secondhand upright can't be measured by the fact that it fits in between the armchair and the standard lamp without overhanging the fireplace! It has to sound good too which may be harder to find in a smaller instrument, though not completely impossible!
Obviously if room is a major factor you can't do much about it! I've been there too! In which case, go for the digital and get the best deal you can.