From the gov.uk website:
Dr June Raine, MHRA Chief Executive, said:
We continually monitor safety during use of all a vaccines to protect the public, and to ensure the benefits continue to outweigh the risks.
Our thorough and careful review, alongside the critical assessment of leading, independent scientists, shows that there is no evidence that that blood clots in veins is occurring more than would be expected in the absence of vaccination, for either vaccine.
We have received a very small number of reports of an extremely rare form of blood clot in the cerebral veins (sinus vein thrombosis, or CSVT) occurring together with lowered platelets soon after vaccination. This type of blood clot can occur naturally in people who have not been vaccinated, as well as in those suffering from COVID-19.
Given the extremely rare rate of occurrence of these CSVT events among the 11 million people vaccinated, and as a link to the vaccine is unproven, the benefits of the vaccine in preventing COVID-19, with its associated risk of hospitalisation and death, continue to outweigh the risks of potential side effects.
[...]
While we continue to investigate these cases, as a precautionary measure we would advise anyone with a headache that lasts for more than 4 days after vaccination, or bruising beyond the site of vaccination after a few days, to seek medical attention.
[...]
We will continue to robustly monitor all the data we have on this extremely rare possible side effect.
So they are still referring to it as a possible side effect. What's changed then? I find this statement ambiguous which is disappointing. I was expecting something more categorical and reassuring. Guess it's early days in terms of data.