BA haven't disabled the ability as far as I know. If you have a restricted ticket, the option will usually be greyed out anyway. BA have been making sure that people affected by the cancellations can change their flights themselves online, but I'm not sure whether this was because of the ash, strike or both.

Either way, it's not uncommon to be unable to change online. Just ring them up, it's no hardship (once you get through to someone, of course...).