The BE (Black edition) are the K APUs. The 5700 isn't a K, it's clocked slower and you can't adjust the multiplier for overclocking, for the sake of saving literally a couple of quid it's hard to recommend using them.
My buddy Idlemans intel spec is quite nice but leans on B grade parts a lot. The H61 mobo is too basic for my liking (and could be missing accessories like sata cables) but it is cheap as is the CPU (granted the OP said future proofing isn't essential). It's still more than £300 though, tis a very tough budget to work on