Honest answer:
Sell your car and get a van.
Yes you may think that's drastic, but honestly that's what you need to do.
Working from a car with wfp is not the same as working from a car with trad.
Using a car with ladders is easy, but using a car with wfp is a right pain.
You can pick up a decent berlingo or partner for under £2k, and get a nice 300 litre system fully fitted by a pro company for under £3k. So that's a total set up for only £5k.
Don't know how much your car is worth, but say it was an old banger only worth £500:
Sell Car: £500
Tesco loan: £4,500
= £5,000
The payments on a tesco loan for £4,500 over 3 years are only £148 per month, so about £34 per week, and you'll have a proper set up. Or keep the car for the missus and pay £151 a month instead.
Say even on a bad day £34 is 2 hours work. I guarantee you that having a van system will save you MUCH more time than 2 hours per week, simply in faffing about filling up barrels and carting them about.
Speaking as someone who knows, as I've been in the same position as you, forget messing about with barrels and backpacks etc. a van system is really the only decent thing to have.
If you go down the backpack/trolley/barrel/car route, I guarantee that in less than a year you'll wish you hadn't waste your time and money on it, and just got a van and system in the first place. Yes its a lot of money to spend, but it's what you need. Take a deep breath and do it properly.
Just my advice.
Also, by the way, backpacks, barrels, and trolleys are heavy. It is hard work doing wfp with them, I'd even say its harder work than trad. In contrast working from a van, where all you have to lift is a hose reel and pole, is so easy its like cheating.