https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/mentalhealth/bulletins/selfharmandsuicidebysexualorientationenglandandwales/march2021todecember2023#glossary
From the source, section "2. Background to the research"
"This question did not collect any information on gender identity. In this release, the abbreviation LGB+ refers to people who described their sexual orientation as "Gay or Lesbian", "Bisexual" or "Other sexual orientation" in Census 2021."
In the glossary:
"Sexual orientation
Sexual orientation is an umbrella term covering sexual identity, attraction and behaviour. For an individual Census 2021 respondent, these may not be the same. For example, someone in an opposite-sex relationship may also experience same-sex attraction, and the other way around. This means the statistics should be interpreted purely as showing how people responded to the census question, rather than necessarily being about whom they are attracted to or their actual relationships.
The sexual orientation groups included in this release were "Straight or Heterosexual", "Gay or Lesbian", "Bisexual" and "Other sexual orientation"."
So I do think they designed this to focus on sexual attraction, leaving any gender theory to the side.
A very sensible approach I would like to see more widely adopted. Some day in The Guardian as well.