failure to pay the finance on the part x is just poor practice on their part - but the risk becomes theirs as the finance company still own the car and are more than welcome to come and repo it if they don't pay - so they will get that paid up pretty sharpish I wouldn't worry.
The failure to provide tyres is annoying, but I suspect they may be just finding out about the cost of the tyres for the 19" TI rims and trying hard to find something cheaper.
As you say it was agreed within the terms of sale failure to provide would constitute a breach of contract - therefore a civil matter, not a criminal offence. You would still be within your rights to complain to trading standards though but essentially the most direct form of redress would be to file with the small claims court.
Sounds a bit drastic but I bet they'd soon pay up if you threatened. Essentially what you would do is pop on down to the local small claims (county) court, I think it is an N1 form you fill in and pay a small sum, used to be around £50, may have gone up. That that is (essentially) it, then then put together all your evidence and await your day in court. A fun day out for all the family.
A trick I've used a couple of times before now, the form is free to download and print, so fill one in, print it, show them it along with a nice cheque written out to Her Majesty's Court Service and see if they cave. They may not and could annoy them to the point where they say fine, take me to court then, and is a calculated risk, but good fun!
If you do decide to claim, which honestly I would advise as a last resort, then ensure you claim for an costs associated with their breach, so things like fuel, telephone costs, postage costs for any documents, time out of work, the absolute lot.
A court can never award you more money that you're claiming for, but they can award you less - so if you claim for everything, they might not award you everything, but if you don't claim for additional expenses they can't award it to you again later.
If you do go for it, as a tip find out what tyres the car comes fitted with as standard, regardless of what is actually on it at the moment, as they're likely to be an expensive branded tyre from factory, then you can claim you were following the manufacturers guidelines on recommended tyres for your vehicle and end up with a free set of good quality boots.