At least some government guidance (the Traffic Signs Manual) recommends a width of 1.5m–2.0m for cycle lanes, noting that anything wider than that tends to get used by motor vehicles as an extra lane. I'm a little confused by your comment on TROs since the ones I've digitised (http://bjh21.me.uk/traffic/) either specify a fixed width of 1.5m (areas B, C, and D) or specify a range of 1.0 to 1.5m (areas A and Z).