No, U174 isn't a UK NATO plug, it's a US/UK/other helicopter plug and essentially the same as the US NATO U93.
A UK NATO plug won't fit in such a socket, it'll be slightly too big. A U174 plug will fit into a UK NATO socket, but it'll fall out easily. The pinouts aren't the same though.
What's the 7 pin connector?
It might have to be done in two stages, UK NATO to U174, then U174 to whatever this 7 pin connector is. Or you could have twin GA plugs in the middle. Or anything really!
Have a look here:
http://www.headsetservices.com/adaptors.htmlAh, I see HF supplied that exact link!
It might be just as cost effective to get a custom lead made up rather than buying the two adaptors leads.
Also then if the helmet is still UK military spec (150 ohm non-amplified microphone), you'll need to change the microphone to a civil spec one (150 ohm amplified) or put an amplifier in circuit (if you're using a LEMO plug, it could be powered off this!). The earpieces should be fine.