Why receiving more frigates does not guarantee a better fleet

Would foreign navies necessarily be better if Britain sold them larger numbers of Loch-class frigates?

Only if they could crew, fund, train, and operate the additional ships instead of leaving them inactive.

More Loch-class frigates would give the recipients additional hulls, but their value depends on whether those hulls can operate. Each navy needs enough crews and funding to keep the ships active. Competent leadership and regular training also matter. Without those conditions, the transferred ships could simply remain in harbour, so the comparison needs to specify both the alternative ships and the recipients' ability to use them.