This is a right pain the arse for you. Having a similarly minded BIL, I understand how you feel - my first instinct would also be not to attend.
I do, however also regularly make an 800 mile round trip to Scotland from the south of England, with small children, and there are ways to make it manageable.
We have everything packed up the night before, and get in the car around 4 / 4.30am. With any luck, your toddler will sleep through again until their normal breakfast, and that’s your first stop. You can get a fair way up the country in that time. Probably whichever one of you is not driving should sit in the back with them, and be ready with entertainment & snacks.
As you’ve got the extra 100 miles, you could consider making a much bigger trip of it, and staying at a premier inn or travel lodge for a night on the way up and down, and finding something to do in those areas. We stopped in Birmingham last year, having broken the back of the journey, went to Cadbury World the next morning, then drove the final 2.5 hours home later that day. You’d just need to figure out the rough area that was good to stop for you.
Assuming the wedding is a Saturday, I’d suggest leaving early on the Thursday, and making it to central Scotland around lunch. Spending the afternoon and night there somewhere cheap. Then make your way up to the ferry on the Friday morning. Friday & Saturday night on the island for the wedding, then leave whenever the ferry time works best. Do the worst of the journey on the Sunday, find somewhere to stay overnight, and have a morning / day out the next day before heading home. If you could stretch to two nights on the way home it would probably be more relaxing.
This way won’t make it any cheaper, but 2 nights on the island and a family trip may make it a bit more palatable.
Ultimately I think you need to suck it up for the sake of future relations with all your in-laws. Obviously the toddler will be very tired and it would it would only be right for you to retire to your room with them (and Netflix / wine / snacks), whilst your husband stays at the party 