No, under say the normal OTL with POD after 1920s the Soviet Union will be unable to invade Britain once it reaches to channel
Without any special events, once Soviets will be able to build up their invasion force to succesfully attack Britain, the British might have nukes.
Of course another questions is if the Soviets can effectively destroy Britain by invading Middle East and India.
That depends on how long you accept between the red army reaching the channel and Sealion attempt.
If the red army is at Calais in 1944, it's likely the USSR has a chance of a successful Sealion by 1975 ( or maybe slightly earlier ), given that equipment devlopment, training and doctrine won't be anything remotely like OTL.
If the Soviets get to the channel Britain would quickly become very xenophobic and reclusive and nationalists would likely get more seats in government, its likely they will be even closer to the United States and you may very well see the UK in Vietnam. Also things like the EU probably won't develop the same, the Anglosphere will be the bulk of western democratic power since all the other colonial powers will be soviet influenced.