It's not the abilities of the state arms that prevent a revolution, but the actions of the state power in providing a safety valve for discontent amongst the less privileged power groups (in this period, the middle classes). Britain avoided revolution in the long nineteenth century through reform - repeal of the corn laws, great reform act, widening the suffrage. Although reform often didn't go far enough for some, it was always enough to reduce the support of any revolutionary movement and so any violent outbreak would remain as a riot (cf Bristol 1831) rather than escalate.
Therefore, what you need to do is remove some of this safety valve activity - have a wider noble class who seeks to protect their interest through pushing away the middle class rather than co-opting them, perhaps? Although butterflies from that social change may well affect Britains pace of industrialisation amongst other things... With that set up, you can introduce a spark of pretty much any kind to turn tension into violence.