I think they are £100, and there seemed to be a fair degree of slop at
full extension which may not be adjustable out.

What width are you cutting?

Non Sliding...
Makita MLS100 will do 136x60, about £100 from Lawson, may be similar
on Tooled-Up, reviews on Screwfix.

Sliding...
Bosch PCM8S will do far wider, about £200 from Tooled-Up, check around
for reviews.

I have not used either saw (or the above suppliers), but to say
branded is expensive is not always true; they are not cheap but they
are known to give a reasonable cut with parts backup. The test for a
mitre saw is making a hexagon picture/mirror frame, cheap either
create a curved cut or one where the angles simply do not meet up - it
is a difficult task, but a good test.

If it is to do rough work then the Argos saw (Challenge?) was
recommended some time ago, not sure if it is still stocked/available.
The cheap end is a highly variable area - the same saw can be
rebranded or even slightly modified, once you get into sliding saws
quality really does count. Most saws say factory setup, that appears
not always true from many reviews of many saws (Makita included).