Alternatively they could be thinking that the 455s are available, so why not run them in to the ground and take the time to get the next batch of 701s properly right before introducing them.
SWR do seem to be struggling with commissioning more units as they were averaging 7-10 days back in November and early December.
Even allowing 3 dead weeks over Christmas and new year, bringing extra units into service during January seems to have slowed almost to a stop.
Perhaps they did easy ones, and now taking longer. But of course aren't alone in struggling to make units stored outside without extensive production from condensation, damp, mildew, etc usable. GWR with 175s, Great Northern 379s weren't exactly parked in an unlock them and switch them on, all ready to go condition either.