Britain and the US are politically, and economically entwined with each other. As a country, keeping core technology at home is tied to defense (don't buy your guns from your competitor). If they don't intend to go to war with the US, then there isn't any real defense loss (I'd also point out that making IP "blueprints" for a core is different from manufacturing the core itself). If Britain were to have a real issue, it would be the US locking down F35 jets they sell to their allies.