I have a Macbook Pro Retina Display 13" model with a 250GB SSD which was brought in January this year. I always knew that there wouldn't be enough storage space for me at 250GB however always planned to upgrade with external storage and eventually replace the hard drive with a larger one. The time has come and now I am deciding on the most cost effective, most stable SSD to replace my original one with. I ideally would like to spend around £150 to £200 and want at least 500GB.
Changing a SSD for a larger one is usually pretty easy. I have done it myself via a kit OWC (macsales.com) sells. We also use cruical.com at work and buy SSDs from them. As for brands and make/model... I've always shopped price as HD/SSD are all made by about 5-6 companies and all are good and should work with any Mac that supports SATA.
You will find good instructions at ifixit.com so you know what you are in for (you may possibly need some specialized "screwdrivers." That guide will tell you.
However as you seem to be in the UK I am unsure if these places sell overseas and/or will be price competitive with a reseller in the UK.
Regardless iFixit is a great resource and will help regardless of where you get the drive.