A 270 is a very small inflatable with huge bottom drag, to overcome fast on flat calm non windy waters will need a engine that delivers full response and power at full throttle to plane faster, that's with well inflated and ballanced sib. A 2 strokes 15/18 will plane it near instantly, but sib has a nervous response with a overpowered engine reving on a small size hull.
Don't know about legal issues in UK, but a Tohatsu 8 is a detuned version of same 9.8 HP older brother. Both are same size and weigh, it's a different engine from a 9.9 though. 170 CC against 250 CC.