After a spat of public criticism and vocal reprimands, it appears that this year's UK Games Expo has revised their health and safety policies regarding COVID-19.
On their official website, the ones behind the expo laid out multiple changes to the venue and to safety requirements for staff and attendees. There are now additional open spaces to allow for social distancing, this includes more space in aisles, more open tables for gaming groups, and 8,000 square meters of open space while waiting in queues. Furthermore, the convention's Bring and Buy program has been suspended for public health concerns, a new ventilation system has been installed that's on par with local hospitals to allow for clean air circulation, and hand sanitizers will be made readily available across the entire convention center.
As for masks and vaccination cards, there is a lot more to parse. Face masks will be required while indoors unless you have a verified medical exemption. If you're over the age of 16 you will need to provide proof that you are at low risk of transmitting COVID-19. This includes submitting to a Lateral Flow Test, getting a negative test result within the past 48 hours, then providing documentation of the test to security before entering the expo. This can be done through several smart phone apps and is generally encouraged to be kept on smart devices as a PDF to work around patchy network coverage or burning through battery power.

Alternatively you can also provide documentation that you have been fully vaccinated either by a vaccination card or an e-mail sent by your healthcare provider. Finally, if you are attending the UK Games Expo from overseas you will need to submit to these tests as well, or provide proof of vaccination as recognized by the WHO a full 14 days before your arrival at the expo center.
Considering that the original policy for the UK Games Expo was a complete lifting of safety regulations towards COVID-19, this is a substantial pivot for one of the largest tabletop gaming conventions in the UK. At least now