I understand that the direction that Bank staff work from an office at least 2 days a week or 40% is directed at all LBG colleagues (apart from those who have a home worker contract or work in specific areas), and therefore applies not just to those who have been home working because of the pandemic. You raise by way of example members being given this direction who have worked from home for 10+ years but who do not have a home worker contract.
The starting point is to assess the terms and conditions of employment of any such member by reference to their written contract of employment and whether, through an implied term, their contract provides that their workplace is contractually their home and if so on what terms.
This may be arguable on the basis that your member relies on conduct since they commenced working at home as showing an implied variation of the original terms, such that they had become a home worker under their contract.
If their written contract is silent in terms of where the place of work is , and importantly how it may be changed ( and I very much doubt this is the case) then it seems to me that long standing conduct of the working from home may mean their contract now provides that their workplace was home, and this might have become a contractual term.
However, I anticipate that their written contract of employment will reserve the right of the employer to change their place of work. This would apply even if they had been based at a particular branch of the Bank for 10 plus years, and I am not convinced it would be any different if they had been working from home for such a period.
I think it very unlikely that if the bank have reserved the right to change the place of work within the terms and conditions of employment, they will be precluded from imposing a requirement to change their place of work on reasonable notice. Hence, they are likely to be able to require them to attend at least two days a week (or 40%) in the office from September 2023 notwithstanding a practice that has been long established. This might be construed as in effect imposing a change in their place of work to the office in question with an allowance for them to work from home three days a week.
Whether, notwithstanding this generic advice in individual circumstances, a member could establish that their contract had been changed through long standing will depend on their own individual circumstances and in particular the basis on which home working was first introduced and has been maintained, and if there is any conduct or positive representation by the bank that suggests that working from home was a permanent position.
As previously advised, though it may not be established in contract, the reason for home working when first introduced may be justified as a reasonable adjustment because of the member’s disability. There may also be circumstances ( I anticipate rare ones) where, though not disabled, when homeworking commenced the member is now disabled and that might now require that as a reasonable adjustment home working continue.
It may also be the case that even though the present circumstances of the member are such that if a request for flexible working were now made the Bank may face difficulties justifying the requirement to work 2 days a week in the office. It may be that such a requirement adversely impacts more on female staff due to their domestic circumstances and may therefore require the Bank to show that the requirement being imposed on all was a proportionate means of achieving a legitimate aim if some cannot comply with the requirement.