At the UK National Archives, a search of the online catalogue for Canada finds 45,576 results and for Ottawa 3,133 results. You can filter the results by time period and various archives beyond TNA.
If you can localize your interest visit the county record office online where you’ll find material in addition to that in the catalogue. Here’s a tiny sample.
The Derbyshire Record Office has an active blog. Searching in the updated online catalogue shows 259 entries for Canada and 12 for Ottawa.
The Norfolk Record Office is active on Twitter. The online catalogue shows 437 results for Canada, 8 for Ottawa.
The Lancashire Archives and Record Office offers “Thoughts from Lancashire Archives podcast”. There are 308 hits for Canada in the online catalogue and 16 for Ottawa.
The Dorset History Centre has an active blog. Find 130 items for Canada in the online catalogue and 2 for Ottawa.
There’s a list of county record offices here, and a longer list from TNA with other archives here.