As pointed out in the other answer, hearings in the Supreme Court are live streamed and recorded. This has also occurred in the Court of Appeal (Civil Division) since November 2018. However, the hearings in these courts are not "trials."
Ordinarily the open justice principle requires that the public be permitted to observe all court proceedings by attending in person. During the COVID-19 pandemic, many courts are upholding this principle by permitting members of the public to access the video conferences in which hearings are taking place. Information about how to do this is generally available from the relevant court's daily cause list.
For example, the daily cause list for the Royal Courts of Justice currently says:
Requests from the media and others, including legal bloggers in the County Court at Central London, to attend a hearing should be made to the County Court at Central London email@example.com or by calling 0300 123 5577.
Arrangements will then be made for you to attend.
When considering the use of telephone and video technology the judiciary will have regard to the principles of open justice.
The court may exclude observers where necessary to secure the proper administration of justice.
While the courts are open to the public, there are restrictions on reporting what occurs. For example, at a trial for sexual offences you might see the complainant give evidence and hear others identify them by name, but victims of sexual offences are entitled to anonymity outside the courtroom. Creating your own audio or video recording of court proceedings is also prohibited. By limiting live stream access to people who request access to individual cases, the court is able to ensure that these restrictions are communicated to and enforceable against members of the public who attend court.