Most of the time you go just for the one morning and then you’re done
That's not my experience and certainly not something that can be relied upon. I think I was only excused for a few hours within the whole two week period and spent several days sitting in an uncomfortable crowded room with not enough chairs for everyone waiting to sit down.
Food and drinks were appalling and you couldn't buy a lunch and a couple of coffees with the allowance they gave you, but it was difficult to go out and get back in the time allowed. Certainly all done on the cheap, but at the same time ridiculously inefficient. if they organised themselves better, they could get it all done with a better 'experience' without costing more.
The general population is on the whole pretty stupid, and easily swayed by emotion not facts and evidence
I also agree with this comment. I spent about 3.5 days involved with two trials over a 2 week period. One was a majority rather than a unanimous verdict because the jury wouldn't agree and it was clear that some jurors were just voting in a certain way to get it over and done with and the second reached no conclusion at all because it was clear we would never get even a majority verdit.
OP, sympathies, you can only write and explain your predicament and ask to defer. Your circumstances might be different if and when you get called up again. I do wonder that, if the situation with the dog is true, eg that you work from home and look after your dog at the same time, would they pay for dog walker expenses if you did serve?