Direct from the CAA's Trainingcomm 2004:-
RULE 5 & EFATO
Misunderstanding of how Rule 5 (Rules of the Air Regulations 1996) applies to practising engine failure after take-off (EFATO) in single engine aeroplanes continues to cause problems for some FIs. EFATO is not considered to be part of “taking off in accordance with normal aviation practice” and so there is no exemption to Rule 5(1)(e). The Rule requires that you are separated by 500 feet clear of other people, vessels, vehicles or structures; it does not mean you cannot fly lower than 500 feet above the ground. The same Rule applies to PFLs. With careful planning and your knowledge of the local area, it is entirely possible to teach PFL and EFATO skills realistically and without infringing Rule 5.
For landing at Farm strips the CAA also publish this guide
http://www.caa.co.uk/docs/33/srg_gad_06webSSL12a.pdf
Though it does not actually talk about a low pass to assess. IMHO I would say this is OK if you
do intend to land but certainly not if you do not have this intention to start with.