Yes it is nice if you get some backpay after waiting for the start of your award.
It can be a bit like saving up, but in reality means that you are missing out for a while on what you should have.
I got some backpay with ESA when I first claimed, my WCA was quick enough but the decision was fit-for-work and it took an appeal and some months to sort out my award, I bought a new washing machine with most of my backpay and had a celebratory feed and booze up with the rest.
In reality though they should do the work capability assessments, and make awards, quickly enough that nobody has to wait long enough to be owed any backpay.
Which is what has happened with your LCWRA, they have done it quickly enough that no backpay is due.
Have a good christmas yourself, but I have to say it must suck a bit not getting your UC payment until the 26th, at least it will be there for the new year.
My ESA is due on the 25th so will be paid on the 23rd.
PS. For those who aren't sure yet this gives the benefit payment dates over christmas and new year,:
https://www.gov.uk/how-to-have-your-benefits-paid