The advice at this, the Compliance stage of the process, is indeed to pay to avoid further charges; the bailiffs part of the sum demanded is £140 the legal compliance stage fee. Submission of an out-of-time Witness Statement (one for each PCN), is not affected by your paying the bailiffs. Incidentally, bailiffs are now legally called "enforcement agents" since 2014.
Whether an OOT WS would succeed is moot, because you have not updated your V5C for 7 months, so you would need a cast-iron reason for not doing so. Have you such a reason ?
If you don't pay, they can visit and add on £235 per PCN, and finally start to take control of your property. Your car is likely first for seizure, because they can only seize goods at the address on the warrant,
plus goods on the highway.
Extract from Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
Goods which may be taken
9An enforcement agent may take control of goods only if they are—
(a)on premises that he has power to enter under this Schedule, or
(b)on a highway.