No, they are the same. Actually, ipostparcels is owned by UK Mail and since UK Mail has recently been purchased by DHL, ipostparcels too is being managed now by DHL.
When you check both UK Mail and ipostparcels websites, you'll see on top it says 'Part of DHL'.
We also wrote an article before that explains in detail the difference between UK Mail and ipostparcels. If you're interested to know more, you can check it out here: