How hard is the HDD drive change?
If you have calm hands, some patience and the right tools, it's pretty easy, actually. Take a look at some guide like the one from
iFixit.
Can anyone recomend a good drive that fits (UK please)
Any current 2.5" drive should fit. Take what seems to offer best price per GB to you. Most drives should easily outperform the stock drive, even if they can't get close to the speed of an SSD.
Is its easy to 'wire' up both drives in the 2011?
Pretty easy. You have to get a dedicated cable (ideally as part of a package with the required tools included). That cable has to be bend a bit, but usually there are marks on it where to bend. Keep in mind that the cheapest cables only support SATA-3Gb/s, so if you ever intend to go SSD on such a cable it won't run at full possible speed or won't run reliably at all.
Is RAID easy to set up? (where it appears as 1 drive)
Apple's hard drive tool box offers what you need and
the process is pretty straightforward. You may have to boot from an external drive to RAID both internal drives together.
Keep in mind that usually it isn't recommended to RAID two drives with different sizes together.
If you want to achieve a higher speed, instead of RAIDing the drives you may want to consider replacing the stock 500GB harddrive with a SSD, which offers a significant speed increase over a RAID setup with the 500GB stock drive.
Prices for a 512GB SSD like the
MX-100 are quite affordable these days.
You could still set up a fusion drive with such an SSD and the new 1.5-2tb drive, but this is discussed controversially. With a 512GB SSD you should be able to keep OSX fully on that drive and enjoy even higher speeds, while halving the risk of disk failure, so I would argue against making a fusion drive with such a big SSD.
In any case you should have a proper backup strategy in place (e.g. at least Time Machine active).
