This touches on two areas of professional expertise for me, so I apologise if this ends up being more detailed than you need.
You can ship just about anything in the UK or EU, it's just a case of properly declaring it and using an appropriate courier if it's classed as being "Dangerous Goods"; TNT, FedEx and UPS all offer DG as an option but you pay about £10 more per item and have to fill in an additional form.
Exactly what classes as DG is defined by the Agreement on the Transport of Dangerous Goods by Road (ADR) which is comprehensive in an impenetrably detailed sort of way.
Carbon Tetrachloride is UN 1846, Class 6.1 (Toxic), Packing Group II (Medium Risk) – see Pg 380 of Volume 1 linked.
Quantities under 100ml are exempted and not considered Dangerous Goods, Quantities under 333KG can be transported commercially under a separate exemption set out in paragraph 1.1.3.6 (Sub Paragraph 1.1.3.6.3 identifies the quantity) subject to appropriate packaging and paperwork being used.
If you personally transport it in your capacity as a private individual Paragraph 1.1.3.1 clause a) exempts you from the carriage regulations entirely.
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Regards disposal the advice to allow it to evaporate to atmosphere is woefully outdated, and doing so deliberately for the purposes of disposal would constitute a criminal offence under the Waste (England and Wales) Regulations 2011.
Release to the environment is also pretty terrible for said environment (specifically groundwater) even in comparatively small quantities, this is why Carbon Tet and other chlorinated degreasers/dry cleaning agents have been progressively phased out under REACH and latterly UK-REACH.
That said, those self-same waste regulations impose a legal duty on your local council to provide a service for the disposal of Hazardous Waste from Householders. So you should be able to discharge the extinguisher into a suitable (Polyethylene or Glass) container, then take it to a HWRC (CA site / Council Tip), arrange to take it to a periodic Amnesty Event, or have it collected from your house (the service varies depending on your council)…
I should emphasize, DO NOT take the whole extinguisher to the Tip, discharge the contents into a suitable container first and take that (ideally clearly labeled with the contents to assist the poor soul who periodically gets sent to catalogue and collect it all) unless you want to end up losing your extinguisher.