Just be careful of what you wish for.
Suppose that a whole lot of caving clubs happen to disagree with something which you are strongly in favour of. These clubs all put out a plea to their members to send in a proxy vote and you, who assumed all was fair and square, find you have lost out to a mass proxy vote. Would you still be in favour of proxy voting?
Proxy voting can be a two-edged weapon - fine if the vote is on your side and everyone agrees - but not so fine if you care enough to go and argue your point with people who listen to you and are convinced by you and then find you lose out on a proxy vote sent in beforehand by a stack of people who were told how to vote by someone and just did what they were told.
BCA does have a one person, one vote, system by using a ballot after a contentious discussion - e.g. the CRoW debate. So, even if you couldn't attend the meeting, you may still have a chance to have your say when it comes to the ballot. It isn't perfect but it's a step in the right direction.
Proxy voting at publicly quoted companies, such as Building Society AGMs, is not a fair example because the large Insurance companies and other shareholders effectively have the proxy votes sewn up. You can go to the meeting and have your say - but you know in advance how the vote is going to go because the big shareholders have already made up their minds.
I don't know what the answer is to the problem but I think BCA is trying to be as democratic as it can, given that many cavers have strong feelings about some things but then say they are not prepared to to spend time coming to a meeting to make sure they have their say.