MEFs as printed on CAA 1/2 mill charts are based on information available concerning the highest known feature in the associated area, including terrain and obstacles and allowing for unknown features.
There is no need to add the mythical 300 ft as that is allowed for - if there are higher obstacles already known about, then they will have been included in the MEF. If not, then the 300 ft will already have been added.
Regarding your question about deg T or deg M, the simplest option is to use deg T for everything, then finally apply variation (and deviation, if necessary) in order to obtain the value you will use on your aeroplane's magnetic compass and DI.