Well, Britain was romanised before and after Anglo-Saxons arrival. It's just that romanisation is a complex and really variant process.
It can be compared to creolization on this regard : what made a roman society was the law, the roman civic conception both imperial and municipal, and religion (imperial cult and later Christianism). On this regard, most oriental provinces were as much if not more romanized than the average western province, even if latin language wasn't dominant (outside urban centers, it was more a situation of diglossy than monopolist : Gaul was still a living language by the IV/Vth centuries).
The Anglo-Saxons invaders didn't simply Conan-ed their way up to full conquest themselves, but both on local and continental influence (especially Frankish, itself hugely Gallo-Romanized, then Franco-Saxon after Charlemagne). Kings of Wessex, for instance, may quite well have been issued from native lines.
People as Ambrosius Aurelianus/Riotomagus (whatever they were the same person, as I think, or not) clearly represented the romanisation of late imperial Britain up to still interacting with continental business.
Now, if you're asking for a romanisation being more close to what existed in Spain or Gaul, you'd need a quite early PoD. These two provinces were quite develloped already on their own (Spain beneficing from ancient trade, for exemple; and Gaul being an agricultural and artisanal powerhouse) while Britain was a bit more undevelloped.
Provincial elites were often the same than before the roman conquest, the romanisation fitting right in the shoes of the previous situation.
Having a more powerful and develloped pre-Roman Britain may just be enough to have more romanized (while, again, I'm not talking of language there specifically) post-Roman Britain : more "compatible" way of life, more presence of pre-conquest roman trade, more important demographics, etc.
It also allow to have more important and numerous urban centers : Roman civilisation and power used them as main transmitters (whatever just fitting in already existing ones, or rebuilding new centers really close to new ones).
As for how? Maybe a slower conquest of Gaul, scaling on more years than just a decade. It would increase both ressources (from roman presence, influence, trade, clientelism) for Celto-Britton states, and need for more strong political ties.
Defeat before Cimbrii? Caesar's head serving as decoration in an Arverni temple? Everything that would delay a bit would be appreciated, but coupling it with an earlier interventionist in the west in the same time would be as well quite interesting (Carthage being crushed in the 1st or 2nd war?)
Your guess is as good as mine there.