You should get a copy of the actual text of the by-law. City hall can send you a copy, but they might ask why. It would be better to go in on your own and photocopy it. Then you will know what your options are regarding exemptions, if there are any, for businesses, farms, etc. Some by-laws have them, others don't.
I've never heard of a 2' limit before, but nothing about reptile by-laws surprises me anymore. There are Ontario municipalities that prohibit all snakes, or all reptiles, or worse. Who makes these by-laws? Our municipal politicians, with consultation from their animal control staff.
For now, I would do nothing. The humane society is not the city. According to the London public library website, the humane society is not responsible for animal by-law enforcement (London Animal Care Centre is). Don't do anything to officially let the city know about your animals, and hope that the humane society leaves it at that. Definitely don't let them back into your house, unless you have hidden your animals (and their enclosures!).
If the city presses the issue, they will should send you something in writing. The odds of them trying to charge you without warning you in writing are slim, unless you have a major incident (i.e. high profile escape). They will send you something saying 'get rid of your snakes or else'. At this point, you will need to decide what course of action to take. The 'or else' usually is a fine, which may be applied on a daily basis until you comply, so make sure you plan ahead and have some place to send your animals quickly if you need to.
Your options, as I see it:
1) Move. Often the easy way out, but not always.
2) Get rid of the offending animals. You can try a temporary arrangement but this may not work. Usually if the city is involved, there has been a complaint from someone nearby, who will probably find out if you get them back.
3) Fight the by-law. This means waiting for them to charge you and take you to court. They may not, if they think it isn't worth it. However, if they do, you could face a costly fight. You can defend yourself, but most people would be doomed without a lawyer. The by-law (at least as it pertains to snakes) does sound quite excessive and I think you would have a decent chance to beat it, but that is just my opinion. It is easy to say this now; harder when the legal bills start to add up.
Since you have already talked to your local politician, I would continue that dialogue (nothing in writing, unless you hear more from the city). See how he or she feels about whether the by-law could be revised. Try to round up some community support (London voters) to convince them that it would be a reasonable thing to do. Don't ask for the moon, just something reasonable and in line with what other municipalities are doing. This is the preferred way to change a by-law, without a court battle! It can be done, but don't think it will be easy...
While writing this, I started doing a search- here is the text of the London animal by-law, effective since 1994: http://www.city.london.on.ca/Cityhal...nimals2000.pdf
Overall, the by-law isn't too bad, but the reptiles part (Class 5 animals for non-venomous snakes & lizards, Class 4 for turtles) is pretty restrictive. The 24" rule is terrible, and also the possession limit of 2 applies to all animals (which includes birds and reptiles, but not fish or dogs!).
Please keep me updated with the progress of your situation, and don't hesitate to email me if I can be of further help.