"At this stage I'm probably looking at just one downspout that I want to use"
It is similar to battery banks.
It does not take much roof area to get a lot of water.
Going with the earlier math,
"For each square metre of house you have, a millimetre of rain will give you a litre."
Neighbor's ~8x14' roof is 10.4 sq meters.
0.1" of rain is 2.54mm = 26.4 liters = 7 US gallons.
Filling a 55 US gallon tank from his little shed only takes 3/4" of rain.
Factor in all the 'usually' conditions, and that 0.1" goes a long ways, because it is getting refilled 0.01~0.1" (3/4 ~ 7 gallons) at a time when the garden does not need watered, so when it needs watered the reservoir is adequately full.
Depending on the location (aesthetically speaking), total number of downspouts, and collected horizontal roof surface area of each downspout, might only need to 'screw up' one downspout.
I live in a slightly >100 year old city that does NOT allow the gutters to be connected to anything underground the city owns. Other than that, I can do anything I want!
There are no 'storm sewers' per say... the 'drains' on the street go into the exact same pipe as the 'toilet sewage'.
I can not connect my downspouts to the city 'storm sewers', but I can feed the downspouts under the sidewalk, through the curb and onto the road, so 40' later they flow into the same damm pipe, because the sewage and storm sewers are the same damm pipe going to the same damm place.
(I did not believe they were the same pipe at the time they told me that, but I now know it is true... don't ask)
And, apparently, I can discharge my downspouts 0.00001" from the floor drains in my basement floor, which are connected to the city's sewage pipes (and obviously my sewage pipes are connected upstream of the floor drains... don't ask).
Sometimes it is better to beg forgiveness than ask permission.