If you have a PEP or ISA with a fund provider then the underlying investment is more likely to be a unit trust. So in these cases you might be better off typing the company name in full. This is because several classes of their shares may be listed on the Stock Exchange. In the vast majority of cases you will be interested in the plain old Ordinary Shares, or Ords. This share will normally have the simplest description and EPIC code.
EPICs have to have a minimum of three characters.
The codes for some companies have two letters followed by a full stop. It is important that you enter the full stop as well, otherwise the code won't work. Sometimes companies change their names. Their EPICs change at the same time. If you suddenly find that an EPIC you used previously doesn't work anymore that is probably the reason why.