CT (Counter Terrorism) - known association with undesireables
BC (Basic Check) - British Citizenship and convictions (if any), electoral register, length of time spent abroad if more than 60 days and places visited that sort of detail.
This will clear you to Class Restricted and Confidential.
If required the next level is DV (Developmental Vetting) (used to be POsitive vetting, but changed name) which involves a full trace of your bank/medical records and an interview with Special Branch or MI5 to go throuhg pretty much everything in your life.
This will clear you to Secret. Top Secret requires further DV and it won't be likely you will ever need this.
Just on the off chance he's genuine, I'll answer this in one easy line
It makes no difference at all.
The reasons being that you can't present any previous Mil vetting paperwork to a civvie and if you go for MOD or Gov work, they'll just vet you again. I have no idea of what their vetting procedure is, for all I know they may well just look up your name and see that you are already cleared through Mil service and accept that, but, they will still physically expect you to submit all of the paperwork for your civvie employment.
This I know for a fact seen as I left the mob being cleared to secret and then had to be vetted all over again within 2 weeks to carry out a contract on an MOD protected nuclear site (reactors not weapons).
I suppose it would go in your favour at the job interview seen as if you have passed vetting with MOD through the Mil, it is logical to assume that you would pass as a civvy.