Word of advice, don't skimp on a UPS.
With that said, how much do you value your data? (If its a lot see previous advice)
Things to consider;
1) Use a UPS calculator such as
http://www.apc.com/tools/ups_selector/index.cfm to help gauge the size of UPS needed.
2) Spec the UPS to accommodate the
maximum combined load of the systems attached to it + some overhead. You don't want it to overload and shut down on you at the 1st hint of load.
3) Consider how many devices you want protected by the Battery and make sure the UPS has enough ports. (The one you have linked above only protects 2 of the ports with the battery)
4) Maximum Runtime
5) VA is not equal to Watts
6) Frequency of power outages, length of power outage, and recharge time.
Not all UPS's are created equal, Some take so long to switch to battery after a power cut that the equipment reboots anyway, and if there isn't a lot of runtime on the UPS it may run out before the devices have finished booting, resulting in two bad shut downs. I currently run 2 of these
http://www.apc.com/resource/include/techspec_index.cfm?base_sku=SMT1500I, the company I work for uses these as the entry level UPS for our clients, the battery lasts around 5 years and they can provide 900 Watts each. (I'm currently running my main rig, a 27in monitor and my NAS off of one of them, should probably move the NAS to the 2nd unit but hey...) Yes they are expensive but what price do you put on your data?
Here is an example that had to deal with recently;
A self-employed neighbour of mine recently almost lost all of the files related to his design business, including past and present jobs, when a brown out blew his NAS's power supply, while we attempted to fix the issue the external esata drive he uses for all his current jobs lost its partition table. As you can imagine he was extremely freaked out by this incident, but unfortunately has failed to head my advice and invest in a UPS or two. He now has two NAS boxes and even more data but he can't see the logic of buying a battery and then replacing it every 5 years...
despite almost losing all his data.
From the information you provided above I would recommend this one
http://www.apc.com/products/resource/include/techspec_index.cfm?base_sku=SMT750I&total_watts=50 there is still a delay when switching between mains and battery but not as long as the Backup-UPS will take. You can get the Kettle lead to UK standard socket adaptors off of Amazon to allow you to plug in devices that don't use a Kettle lead, this unit will also provide a cleaner power supply to the devices which is good, and it can take a network management card if you are feeling flush in the future :D. In my personal opinion I wouldn't trust the Backup-UPS with network equipment let alone a Computer or NAS.
This is one of those complicated topics that a lot of people overlook until it is too late, so hopefully I have provided you with some helpful information.
