Parabellum - not so. The rule stated:
5 (1) The prohibitions to be observed are
(a) An aircraft shall comply with the low flying prohibitions set out in paragraph (2) subject to the low flying exemptions set out in paragraph (3).
Certain offences are "absolute". This means there is no "mens rea" requirement. So whether you intended to break the rule or not is not relevant and you have no defence here. As you can see, Rule 5 is such an "Absolute rule".
If you are closer than 500ft to a person, vessel, vehicle or structure in the UK (without specific authorisation from the CAA in writing, see 5(2)(b)), there is no question of whether you meant to or not. The only defences are that you are within one (or more) of the exemptions to the prohibition created by Rule 5(2)(b) contained in 5(3) (such as being in the process of take-off / landing (5(3)(a)(ii))).
If you are forced down by weather and not in the process of landing then coming within 500ft of a pylon (for example) breaks the rule in 5(2)(b). The weather conditions do not bring you within the scope of Rule 5(3) in any way that I can see. The weather conditions you may cite to the Court to take into consideration when sentencing (after your guilty plea); or CAA may decide not to prosecute on grounds of public interest but that is entirely for their discretion.
Let's see if this post gets deleted like my earlier observation