SC has covered it, in general it is most cost effective to join a scheme even if you only do three notifiable jobs because the BC procedure is so excessively expensive and inconvenient (so as to make you join a scheme!).
Stroma is the cheapest, Certsure and Napit the most popular, the others are more limited.
Read the approved document P that explains the legal side of things, the individual scheme requirements are buried somewhere in their websites.
Most common
Ascertiva Group Limited, NICEIC,
www.ascertiva.com,
Supporting contractors in the building services sector | NICEIC - http://www.niceic.com/, 01582 531 000
ECA Certification Limited, ELECSA,
www.elecsa.co.uk, 0845 634 9043
Both of the above are now Certsure
These two are also common:
NAPIT Registration Limited, NAPIT,
www.napit.org.uk, 0845 543 0330
Stroma Certification Limited, STROMA,
www.stroma.com, 0845 621 1111
These are also schemes that provide the same service:
Benchmark Certification Limited, Benchmark,
www.benchmark-cert.co.uk
0238 051 7069
Blue Flame Certification Limited,
Certification - Certification - http://www.blueflamecertification.com/
0845 1940 31, (possibly via NAPIT)
Building Engineering Services Competence Assessment Limited, BESCA
www.besca.org.uk, 0800 652 5533
Oil Firing Technical Association Limited, OFTEC,
OFTEC - Oil & Renewable Heating Technologies - http://www.oftec.org.uk/
0845 6585080
More geared to heating