If you google ber assessors it will throw up lots of people - just call the cheapest and ask them to do it. I don't think any one actually takes the BER situation seriously unless the house is so efficient that it will actively attract punters when it's worth paying someone to do a decent innnnnnspection. Most purchasers have a reasonable idea of what a BER assessment for any given property will be - the older, the worser, the newer, the better
If you want to be more cautious, ask your estate agent for their recommendation.
mf