As of July 2014 the only actively marketed AlertMe equipment in the UK is Hive from British Gas.
At the time of writing there are no officially supported accessories for the Hive system.
Energy Systems are no longer available direct from AlertMe, which is a shame, because they were a bargain.

Security Systems are no longer available from AlertMe or British Gas since BG ceased development of Safe & Secure.
Accessories are available from British Gas in accordance with the price list and you can order by calling 0800 980 8593.
You won't be able to order a new Hub or bundle package.
Both AlertMe and British Gas will replace faulty equipment under the terms of their warranty.
Contact AlertMe on 0845 200 2874 or support@alertme.com.
Contact British Gas 'Geek Squad' on 0800 980 8614.
Second-Hand
Second-hand equipment may be the way many people go; there are just a few things to consider:
- The seller will have to ask AlertMe or British Gas to de-register their system.
- The system MUST have its unique ID code sticker (the little 'ABC123' code).
- Security kits (cuboid hub with an orange logo or rectangular hub) should normally be supplied with a SIM card.
- Security kits (cuboid hub) with a clear logo on top do not have battery backup or SIM capabilities.
If you are buying a British Gas branded system, you must work with the seller and BG to ensure the system is swapped over. AlertMe can't do anything about these accounts and you will need to speak to the British Gas 'Geek Squad' team on 0800 980 8614. Their attitude to resales is unknown at the moment, so good luck!
I Have My System
The following only applies to AlertMe systems.
Once you have a de-registered system, you will need to connect it to your AlertMe account. As many second-hand systems will be lacking documentation, there are a couple of links you need. If you already have an AlertMe account, simply start the setup process here:
https://www.alertme.com/setup
If you don't already have one, or your main account already has a system associated with it, you can create a new account here:
https://www.alertme.com/user/register
From that point simply follow the on-screen instructions to set up your system.
Alternative
There is an alternative to all this. You could purchase a Lowe's Iris system from the US.
I haven't tried it, but the system is also based on AlertMe, many of the accessories are identical and I don't see any reason why UK SmartPlugs wouldn't work with the US hub. They retail with no restrictions from any Lowes store (US equivalent to B&Q) and can be imported for about £100 for a Safe & Secure Kit (all taxes and shipping included).
Again, I've not tried and don't know anyone that has, but it would be a great experiment!