UK is technicall a 'clean' country and arrivals would not be subject to transit screening and you normally deplane into the departure concourse B or Z. That said any liquids you buy in LHR will be packed in the 'STEB' (a special tamper-proof bag) which you can take thru transit security without any issues.

https://transport.ec.europa.eu/trans...ls-and-gels_en