The UK, like the US, requires all passengers to clear security to ensure its own standards are met. Consequently, whether you take an airside or landside route you will near to go through that process.

There's nothing particularly weird about Heathrow in that respect, and in fact it's potentially easier to transfer there than in a US airport given that at the latter you also have no option but to clear immigration.

In terms of your question: 70 minutes is possible with a lot of luck on your side and knowing exactly what you're doing and where you're going, but really not advisable given even on a single ticket a cross-terminal MCT would be 90 minutes.