Abandoning conquered lands, while clearly not unheard of in Rome (see Germania after Teutoburg, although the word "conqured" could be overstatement in that case), was see in extremely poor light.
Nero's already less than stellar legitimacy among the senatorial order would plummet further down.
That could have major consequences, which I am not able to detail, for the internal politics back home in Rome. A place where you expect significant ripple effects is Judaea, where you could imagine even harsher Roman repression in the earlier phases of the revolt, as nero would need a victory to show.
What this does to Vespasian's career and the chances of the Flavian dynasty to rise is up in the air. The Julio-Claudians are very likely still doomed, and their post-Augustus legacy even less well regarded than IOTL. I don't think that anyone could restore the Republic at this point (the military isn't going along with anything of the sort), although it is possible that this is considered by some.
A weaker position for both Vespasian and the Empire in general could cause a longer, bloodier civil war. Whoever comes out on top would have a lot more work ahead to regroup and consolidate, although, as Agricola noted, having three-four legions more on the mainland is likely to prove a long term advantage.
In general, I think the scenario is not very likely. The Romans did not abandon rebellious provinces if given a choice (both Hadrian's abandonement of Mesopotamia and Aurelian's retreat from Dacia occurred under vastly different circumstances; the only parallel here is Germania under Augustus, where Roman control was, I gather, a lot shakier than in Britannia to begin with) , they rather devastated and reconquered them if they could.