The following comments are based on my experiences on FSM/24 up until mid-1997 so may be slightly out of date.
The appointment of someone to a BSI Committee is usually because the organisation you work for has been invited to supply a representative. Occasionally former members who have left the organisation they represented to go elsewhere or have retired are invited back because of their experience.
Each Standard has a 'top' committee which finally approves BSs and oversees the work of the sub-committees which may be set up to write particular parts of a standard. The top committee also liaises with CEN and ISO on international standards and appoints representatives from the BSI committee to give the UK's view at CEN and ISO meetings.
There is a full-time BSI staff member who is the Committee secretary. They are responsible for calling meetings, issuing the Agenda and all the paperwork, taking the Minutes and circulating them and reminding the Chairman if things are not being done correctly in meetings. Paperwork can be extensive - each separate item is individually numbered for easier reference before, during and after the meeting. It was not uncommon for the papers to form a pile 2 to 3inches (50-75mm) high when placed on my desk!
Most BSI Meetings are for a day. Time commitment varies and depends much on the needs of your organisation for internal consultation and reporting back. While representing Fire Research, I reckoned on two to three days to read through the documents, consult anybody else I needed to, a day for the meeting and another day to report back to colleagues as I felt necessary. Meetings were 3 or 4 times a year
The meetings are mostly held at BSI HQ which is almost literally on top of Gunnersbury station. This is particularly convenient for the BSI staff as they are then close to the paperwork. But if for some reason members agree to hold the meeting elsewhere, then they can do so.
The size of a committee varies depending on the organisations who are represented. BSI tries to get a balance between the trade supplying the items under consideration and users and official bodies who might get involved with such items. The one I was on had abut 3 dozen members but it was rare to get every single one along to a meeting.
Yes - individuals can make a difference. Very trite example - we were looking at proposed ISO standards for hosereels which included a test on water flow through the hose. The one for 'layflat' type hose (as opposed to the round rubber hose we use in the UK) completely failed to mention that you would need to unwind the 'layflat' hose completely from the reel to carry out this test - and I was able to point this out with a subsequent alteration to the standard before it was published.