There is no stipulation of over 100 years age anywhere in the firearms acts. The stipulation is pre 1939 (or pre WW2).

Part I: Old weapons which should
benefit from exemption as antiques
under section 58 (2) of the Firearms
Act 1968

8.5 Pre-1939 weapons to benefit from
exemption as antiques are as follows:
a) All muzzle-loading firearms;
b) Breech-loading firearms capable of
discharging a rimfire cartridge other than
4mm, 5mm, .22 inch or .23 inch (or their
metric equivalents), 6mm or 9mm rimfire;
c) Breech-loading firearms using ignition
systems other than rimfire and centrefire
(These include pin-fire and needle-fire
ignition systems, as well as the more
obscure lip fire, cup-primed, teat fire
and base fire systems);
d) Breech-loading centrefire arms originally
chambered for one of the obsolete
cartridges listed in Appendix 5 and which
retain their original chambering;
e) Shot guns and punt guns chambered for
the following cartridges (expressed in
imperial measurements): 32 bore, 24 bore,
14 bore, 10 bore (25/8 and 27/8 inch only),
8 bore, 4 bore, 3 bore, 2 bore, 11/8 bore,
11/4 bore and 11/2 bore, and vintage punt
guns and shot guns with bores greater than
10. It also includes vintage (pre-1939) rifles
in these bores.

Note (i) – The exemption does not apply
to ammunition, and the possession of live
ammunition suitable for use with an
otherwise antique firearm may indicate that
the firearm is not possessed as a curio or
ornament.

Note (ii) – The exemption does not apply
to firearms of modern manufacture which
otherwise conform to the description above.
For these purposes, “modern manufacture”
should be taken to mean manufacture after
the outbreak of the Second World War in
1939. Fully working modern firing replicas of
muzzle-loading and breech-loading firearms,
for example those used to fire blanks by
historical re-enactment societies but capable
of firing live ammunition, must be held on
certificate. This includes replica pieces of
ordnance that are to be fired; some replicas
have been produced with a true bore size of
just under 2 inches, thus enabling possession
and use on a shot gun certificate, but with
significant counter-boring at the muzzle to
preserve the necessary appearance of external
visual authenticity.

http://police.homeoffice.gov.uk/publ...s-Guidance.pdf