Looking at the legislation, specifically paragraph 2, (9) b of the poisons act 1972, it states
9)For the purposes of this section, a substance or mixture is “excluded” if—
(a)it is medicinal, or
(b)it is contained in a specific object.
Section 11 defines a specific object is defined as:
11)A “specific object” is—
(a)an object that, during production, is given a special shape, surface or design that determines its function to a greater degree than does its chemical composition, or
(b)an article that contains explosive substances or an explosive mixture of substances designed to produce heat, light, sound, gas or smoke or a combination of such effects through self-sustained exothermic chemical reactions, including—
(i)pyrotechnic equipment falling within the scope of Council Directive
96/98/EC on marine equipment, and
(ii)percussion caps intended specifically for toys falling within the scope of Council Directive
88/378/EECconcerning the safety of toys.
a fuel tab could be construed as a specific object that produces heat through an exothermic reaction, so it might be excluded.
Link to legislation