Whirly,
Rule 5.1.c refers to the 1500ft and congested areas.
There is a way round it, certainly for a twin engined aircraft. You can apply to the CAA for an exemption to the rule.
Not surprisingly, it costs money. If the CAA agree, an exemption letter is issued (you must have it in your possession whilst flying said mission). The exemption lays out date of flight, type and actual registration of heli, pilots name, and any other conditions, such as the requirement to operate under Class 1 / Performance 'A' conditions, so that an engine failure won't jeopardise anyone's safety.
We often have to do this, thankfully the company pays the £90 required for each exemption.
