Re cutting 4.3bn from the benefits bill, they could do this from the cost it takes to run the benefits system - I imagine it's much like the managerial issue other posters are speaking about in regards to the NHS.
I have been working for DWP for about a year now after many years working in the private sector. I have never worked somewhere so badly managed and inefficient. You simply wouldn't get away with it in the private sector where you're accountable to actual people with skin in the game - but when it's the public purse it's a free for all.
Staff retention ridiculously low due to poor training so wasted paying 2 months of wages for people to not actually do one day of the job. I've been told off for working too quickly because they want the work to last long enough to get overtime, piss poor training done over teams with highly paid "trainers" paid a FORTUNE to read off a PowerPoint screen they didn't even make because they haven't actually done the job themselves that they're training us for, ridiculous hour long meetings with up to 2500 sitting watching big wigs patting themselves on the back about a job well done, ridiculously poor work ethic culture with everyone adopting a "not my problem" and "let's kick the can down the road" attitude - hiring us all through an agency so paying the agency fees on top of the cost of wages, tax, NI, etc - and we could easily run as efficiently (which isn't very due to aforementioned poor training and bad management and terrible work ethic) on a quarter of the staff! In our site alone 120 people were brought in over the course of a week - no one was even interviewed, just checked they had 5 gcses and given the job, and we were all handed laptops, monitors, keyboards, wfh equipment, etc like they were fucking tic tacs with absolutely zero checks that we were remotely competent at the job.
We are now working with really vulnerable people with zero training and support and are thrown under the bus constantly - the decent, hard workers who actually care about the people we're working with are leaving in droves (meaning all the money invested in their training, useless as it was, is wasted - plus repeating the cycle hiring more to go through the same), leaving the work shy, dossers who don't give a shite because it's so easy to skive about and do fuck all, which enivetably is just putting vulnerable people at risk.
Don't even get me started on the amount of managers that exist just for the sake of being managers, literally creating jobs for themselves.
The first 6 months of this job was literally just fixing multiple different mistakes on a nationwide scale - just clearing the backlog, not even fixing the problem, which is just going to repeat itself over and over and then a new team will be hired to clear up the mistakes again when the backlog gets long enough. The civil service is literally just creating jobs for itself and pissing money up the wall left right and centre. All the while whinging about pay rises - in my opinion we're already overpaid for what we're doing. Never mind the hypocrisy of how things are run that you just wouldn't get away with in the private sector, including major breaches of GDPR and the mishandling and losing of people's information and a complete disregard of their duty of care to vulnerable people.
Cutting numbers and increasing efficiency, employing competent staff and prioritising staff retention of good workers, and cutting out the middle man of recruitment agencies would go a long way in reducing the benefit bill.