I think you'll find most hour builders with commercial ambitions will log Hobbs
Then they are committing fraud. Sorry, that's just how it is.
As for formal guidance, it's easy to look up:-
From the ANO 2010:
Personal flying log book
79 (1) Every member of the flight crew of an aircraft registered in the United Kingdom and
every person who engages in flying for the purpose of qualifying for the grant or
renewal of a licence under this Order must keep a personal flying log book in which
the following information must be recorded:
(a) the name and address of the holder of the log book;
(b) detailed information about the holder's licence (if any) to act as a member of the
flight crew of an aircraft; and
(c) the name and address of the holder's employer (if any).
(2) Detailed information about each flight during which the holder of the log book acted
either as a member of the flight crew of an aircraft or for the purpose of qualifying for
the grant or renewal of a licence under this Order must be recorded in the log book
as soon as reasonably practicable after the end of each flight.
(3) The information recorded in accordance with paragraph (2) must include:
(a) the date, the places at which the holder of the log book embarked on and
disembarked from the aircraft and the time spent during the course of a flight
when the holder was acting in either capacity;
(b) the type and registration marks of the aircraft;
(c) the capacity in which the holder acted in flight;
(d) information about any special conditions under which the flight was conducted,
including night flying and instrument flying; and
(e) information about any test or examination undertaken by the holder of the log
book whilst in flight.
(4) Information about any test or examination undertaken whilst in a flight simulator must
be recorded in the log book, including:
(a) the date of the test or examination;
(b) the type of simulator;
(c) the capacity in which the holder acted; and
(d) the nature of the test or examination.
(5) For the purposes of this article, a helicopter is in flight from the moment the helicopter
first moves under its own power for the purpose of taking off until the rotors are next
stopped.
Meaning of in flight
256 (1) An aircraft is deemed to be in flight:
(a) in the case of a piloted flying machine, from the moment when, after the
embarkation of its crew for the purpose of taking off, it first moves under its
own power, until the moment when it next comes to rest after landing;
(b) in the case of a pilotless flying machine, or a glider, from the moment when it
first moves for the purpose of taking off, until the moment when it next comes
to rest after landing;
(c) in the case of an airship, from the moment when it first becomes detached from
the surface until the moment when it next becomes attached to the surface or
comes to rest on the surface;
(d) in the case of a free balloon, from the moment when the balloon, including the
canopy and basket, becomes separated from the surface until the moment it
next comes to rest on the surface; and
(e) in the case of a captive balloon, from the moment when the balloon, including
the canopy and basket, becomes separated from the surface, apart from a
restraining device attaching it to the surface, until the moment when it next
comes to rest on the surface.
(2) The expressions 'a flight' and 'to fly' are to be construed in accordance with paragraph
(1).
My bold.
I have to correct myself on one point however - the "intention of flight" obviously was dropped at some point. There was a pillock in Stirling around 1997 who after flying an unlicenced microlight in the middle of a public park (and crashing it into a tree) got off an illegal flying charge because he claimed he'd not intended to fly and just got accidentally airborne. Possibly the wording was quietly changed for that reason.
G