Hi Joe - i've recently gone through the same process so maybe I can
offer some useful info.

The regulations regarding replacement windows date back to 2002 and as
I've owned my house for longer I could probably have got away without
a building control/fensa document as I know of several instances where
people have simply written a declaration, at the point of sale, that
their windows predate the regulations coming into force.

That said I also know of one instance of someone who installed windows
without getting the the documentation sorted out and was subsequently
'caught out' when someone contacted the local building control office
(the guy was basically a bit of an arse and I think he rubbed a
neighbour up the wrong way).

In practice I think thats a bit of a rare occurrance but in the end I
went down the road of applying for a building control certificate, in
part because its my experience that solicitors do now routinely ask
about double glazing and also because the certificate was only £70
from my council (newcastle) - not alot to pay when you think the house
can now be advertised as having new double glazing that meets current
regulations for safety and thermal efficiency.

With regard to the design of windows you'd like to have installed the
guidance notes I received from the council boil down to the following
-

1] the glass has to meet the new requirements for thermal insulation -
standard double glazing doesn't do this; there are some alternatives
but commonly this means getting double glazed units that are
constructed with pilkinton K glass

2] If you have have existing first floor means of escape (an
unobstructed openable area of at least 0.33metre squared, not less
than 450mm high and wide, with the bottom of the opening between
800-1100mm from the internal floor level) then you you have to keep
it.
If you have windows that don't meet that criteria then while you are
strongly recommended to provide a means of escape when you are fitting
replacement windows this is not a requirement of the regulations.

3] Glazed areas within 800mm of floor level should be either toughened
or laminated

4] If your old windows have trickle vents for the puposes of
ventilation, your replacement ones do as well, otherwise they are not
a requirement for replacement windows

There are additional instructions, with regard to having proper
structural support, particularly where replacing a bay window. This
wasn't a factor for myself so I didn't look into it.

I installed two 'means of escape' windows out of nine installed on the
first floor. When the building inspector came round he was in the
house for two minutes, had a cursery look around, and informed me i'd
get my certificate in the post within a couple of weeks. I would say
the main thing he took note of were the pilkington k stickers on each
double glazing unit, but he did also comment approvingly on their
being a fire exit on the first floor.

What you might consider doing is having the top openers you desire,
and at least one window, perhaps at the rear of the building, where
the larger lower window also opens to offer a fire exit. I've done
this on a previous house and it did not look out of place.

It's also worth noting that all the fire exit windows i've had fitted,
in my current house and previously, have been top openers - there may
be practical reasons why the guy suggested side opening fire exits but
I don't think their are regulatory reasons.

Hope this helps.