The total program cost is £5.2 billion.
Of this, only £3.9 billion is the ship cost.
Another £1.3 billion was development costs, which is already spent & gone with no possibility of recovery.
Thus, you have £3.9 billion, less the £1.25 billion already contractually obligated, and you get a maximum "theoretical savings" of £2.65 billion.
However, you must then look at whatever cancellation penalties the overall ShipCo contract (the agreement between the UK government and the shipyards that was pushed through before the first contracts were placed) has, etc.
As of right now, at least half the program cost is already spent... cancellation would mean every £ was a total waste.
Any hypothetical "second-best solution" whould be a complete start from the beginning, with all-new development coasts, etc.
Or you could cancel the lot without any Invincible replacement, and place your trust in the security of the UK in the "warm, fuzzy feeling that loving the whole world and not threatening anyone will provide".
Do you really believe in the "harmless inoffensiveness" doctrine?
You know... the one that says "the only reason anyone attacks someone else is because they feel threatened, so if we don't have anything that could threaten anyone (like a military) no one will ever attack us"?