Mission accepted:
First: Have Britain leave NATO so they would actually need a strategic bomber force instead of just task-sharing their deterrent strategy with the Americans.
Second: Have them realize that -despite the need for runways - aircraft are much better for carrying A-bombs then ballistic missiles. Have them realize that in the 1970's at least 5 years before the Americans did and re-instated the B1 project. Of course, Britain no longer part of NATO and relations with the US being pretty sour, they might just HAVE to go with airplanes as they can't get access to the Polaris rockets they depended on for their nuclear deterrent OTL. So airplanes might be their only choice, at least until they sunk enough time and money - especially money- into their own ICBM program to actually see their first rocket becoming operational.
Third: Out of necessity have them team up with France who already left the NATO in 1963 and since developed its own, albeit minimal strategic bomber program in the Mirage IV. As France and Britain already have experience in co-developing aircraft, the new deterrent bomber will inevitably be a BAE-Aerospatiale co-production, as will probably the new Franco-British next-generation cruise missile.
...(insert some sneaky remarks here on the fact that French missiles did so well against British warships in the Falkland crisis)...
Fourth: Be prepared to drastically reconsider all programs underway once the Iron Curtain comes down....