Repairs are exempted from Building Regulations.

A typical spec for a sealed unit would be something like "4/16/4 K-
glass, 300*200mm"

The numbers mean 4mm plain glass, 16mm air gap, 4mm plain glass - so
overall 24mm thick.

K-glass means that the inner pane will have a transparent heat
reflective coating (and stickers on it to show you which face should
be inwards/outwards when fitting it).

K-glass is standard nowadays and the extra cost minimal.

It's best if you remove the original unit and measure it. Measure the
overall thickness accurately to calculate the air gap.

It's normal practice for the glazing unit to be 5mm smaller than the
rebate in the frame in each direction, and to stand up on little
rubber spacer blocks - but if you're measuring from the original
glazing unit you won't have to worry about this.

You may find that the wooden glazing beads are in a pretty tatty state
(or will be after you've prized them off) and need replacing. Beading
can be suprisingly expensive - if you the facilities to run up your
own, so much the better.

As far as fitting the glass, butyl rubber glazers tape is the stuff to
use now. It's a non-setting, thick (typically 10*3mm) and very sticky
- position it carefully between glass and timber to form a good
weather seal.

Wickes sell it, or if you're lucky your glazer will give/sell you a
bit.

Best going to a small local glazier that's happy to make up just 1 or
2 units for a customer.