I think Spence wasn't REALLY happy in his new placement; he also seemed to maybe come around to some of the reasons behind Boyd's domineering/rude/outside-the-rules behavior after Boyd tried but failed (with Spence's halp) to save Linda Cummings from the rooftop falland recognized that he'd been spending years trying but failing to save his own son (which Linda forced him to face again in that episode). I think he realized how much he cared about the people on the team, and Boyd would have gladly welcomed him back as a valuable asset, and someone he truly liked & cared for, despite his gruff attitude at times.

to answer my own question ,at the start of series 9 the new policewomen is critical of Spence for running back from the NATIONAL CRIME AGENCY when BOYD asked.