The 1505 system is an extremely capable surround sound system and excellent value for money but it is ultimately an entry level surround system - you may feel coming from your KHT2005.2 system that this may not be quite the 'upgrade' you wanted?
For example the 1505 system in comparison to you KHT2005.2 would be a step down in terms of performance, power-handling and overall output, and you'd be moving away from the Uni-Q driver array with all of its technical virtues, to a conventional tweeter and driver configuration - still great, but it will lack the larger sound stage and dispersion of the Uni-Q 2005.2 satellites that you will have been used too.
For your budget the 1505 system is one heck of a system and we would suggest that if your budget is set to £500 - £600 that you organise an audition at your nearest KEF dealer, so you can be sure it is the right system for you. However, if you could stretch your budget, we would suggest looking at the KHT3005SE system. This system would provide you a significant 'upgrade' with greater sonic performance, clarity and detail over the KHT2005.2 system.
There is another alternative solution however, and that is to simply replace your subwoofer, as I presume the KHT2005.2 satellites are fine. Currently, you could look at the Q series Q400b subwoofer, (20% off as part of the KEF trade-in 50th Countdown promotion) this would be a cheaper solution and still integrate perfectly with the satellites.
There are a lot of options available to you, so whatever you decide we wish you the best of luck!