Virtual machines are suboptimal for asterisk, because Asterisk a real time application.
My solution was to use small mini-itx systems with solid state disks and/or compact flash for my phone system and my radio system. My main file server is also mini-itx. The 2 asterisk systems draw less than 30W, and the file server draws less than 50W. This is 0.11 kWh.
A kill-a-watt meter was used to make these measurements. These can be had for under $30.00 each. I don't know if they make a version for UK 230/240V mains voltage (BS 1363 plug) though.
I have not had any problem with running Asterisk on a VM. We use it as a PBX and as a hub for app_rpt. It's running chan_irlp and chan_echolink. Maybe it's the host system or VMWare but it's working quite well and has been for months. 73, Steve N4IRS