The M&S Cash ISA accepts transfers in, so there is no reason why you can't transfer the Pru ISAs into the M&S ones.

You will need to ensure that the transfer is handled completely by M&S and you don't withdraw the money from the Pru accounts yourself otherwise it can't be classed as a transfer.

The relevant form is on the right hand side of the following page...

http://money.marksandspencer.com/save-invest/cash-isa/overview/

Hope this helps...