Really there are only two choices. Dexcom and Abbott Libre. Abbott Libre isn't proper CGM in that it doesn't transmit readings to a receiving device. you have to scan the reader over the sensor. You still get a continuous trace though.
Abbott Libre is cheapest to get started on, but has a waiting list as long as giant centipede. It's taking three to four months for people to be allowed to buy it.
Dexcom costs a bomb to get started on (£1075 for the G4) and is about to be superseded by a new release, with no details of any upgrade or replacement path for anyone who buys now.
Alternatively, you can source a dead Dexcom transmitter, bring it back to life, build an xDrip and download some software onto an Android phone to create a DIY CGM that uses Dexcom sensors (which can be bought individually from some srouces) and you've got a CGM system for about an initial outlay of about £60 plus the sensor.
Pricing can be found here:
http://www.diabetes.co.uk/forum/threads/cgm-fgm-price-comparison.75106/
A lot of discussion of the Libre can be found here:
http://www.diabetes.co.uk/forum/threads/freestyle-libre-on-sale.65900/
In Europe, the reader algorithm on the Libre is better than that on the Dexcom, so the delay in readings is much less. The xDrip resolves this problem, but you have to build it yourself.
Whilst the Libre is great, if you need the alarms when glucose levels are going low, then you'd need to use the Dexcom or xDrip.