Visa: USA to UK, Short Visit for Business Meetings & Tourism

Canadian citizens do not need a visa for the UK, either for tourism or business, or a combination of them. As long as it's just meetings, and you are staying less than six months, you will be fine. Your country of residence is irrelevant.

According to government websites you should bring a passport-sized photograph with you. You should probably get a letter saying what you will be doing in the UK from your company, to be on the safe side, to show that you won't actually be working for money over there. However it's rarely needed. A company of any size with experience of overseas work will know what to write.

While in the US it seems to matter, the fact that

I won't be receiving any sort of payment from a UK source during my stay.

does not effect the UK governments attitude or not. They consider paid work paid work, regardless of who is paying. You can find various questions on this site from people (mostly from the US) that have come afoul of that. Even volunteering is fairly heavily restricted.

You can find out information in general about the UK visa regime, including a summary of what you can do on a visitor visa, where you can get one (only outside the UK and in advance) and who needs one (not Canadian citizens) on the offical government site.

The full rules for visitors have this to say on business activities:

Business – general activities

5 A visitor may:

• (a) attend meetings, conferences, seminars, interviews;
• (b) give a one-off or short series of talks and speeches provided these are not organised as commercial events and will not make a profit for the organiser;
• (c) negotiate and sign deals and contracts;
• (d) attend trade fairs, for promotional work only, provided the visitor is not directly selling;
• (e) carry out site visits and inspections;
• (f) gather information for their employment overseas;
• (g) be briefed on the requirements of a UK based customer, provided any work for the customer is done outside of the UK.

Which sounds like it should cover most of your plans. Note that there are more permitted actitivies, these are just the ones that seem relevant to your current trip. Your tourist stuff is fine too.

So in summary, based on what you described here, you don't need a visa for this visit to the UK. You will undergo a landing interview when you attempt to enter the UK, where they may ask about your plans. If you want to reduce the scrutiny of this interview and the chances of being turned away, you do have the option of applying for a visa anyway.

No need for Standard Visa.

See: https://www.gov.uk/check-uk-visa/y/canada/work/six_months_or_less

If you come for other business or academic activities, you can stay in the UK for up to 6 months without a visa, but you can only do certain academic or business-related activities.

The list of activities are the same as the Standard Visa, and can be found here: https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rules