Under common law principles, this may not constitute a theft because the suspects are not carrying the products "away" from the owner's premises.
But this does seem to be a theft under the Theft Act of 1968:
(1) A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it; and “thief” and “steal” shall be construed accordingly.
(2) It is immaterial whether the appropriation is made with a view to gain, or is made for the thief’s own benefit.
(3) The five following sections of this Act shall have effect as regards the interpretation and operation of this section (and, except as otherwise provided by this Act, shall apply only for purposes of this section).
As I read it, the main question is whether this activity constitutes "appropriating." But the statute defines "appropriate" to mean "any assumption ... of the rights of an owner," which seems to be what's happening here -- one cannot donate property unless she owns it, so placing it in the donation bin is an assumption of the owner's rights.
So if it's an appropriation, the rest seems to fall into place neatly:
- That appropriation is dishonest because the activists do not actually have any right to the property.
- The property belongs to another.
- The activists' hope is that some third party will take it away from the owner.
- The activists' lack of benefit provides no defense.