This is assuming your using the fighter from Scotland and the Bomber from London on Italy’s navy.
UK 4 inf, 1 mech, 5 AAA, 2 Fighters, French (2 Inf, 1 Fighter)
Transports - 2 inf + tank + art (Assume subs missed DD and TT at New Brunswick)
Buy 2 Fighters round 1.
If Germany bombs London for 20 and the AB for 6; UK can purchase 3 inf, 2 art.
Total Defense: 11 inf, 1 mech, 3 art, 1 tank, 5 AAA, 5 Fighters
Total Attack : 10 Inf, 9 Tanks, 1 art, 5 Fighters, 5 Tactical, 4 Bombers.
100% Avg units left for attacker 16
Now lets assume that the 2 subs take out the TT at New Brunswick.
Total Defense: 10 inf, 1 mech, 2 art, 1 tank, 5 AAA, 5 Fighters
Total Attack : 10 Inf, 9 Tanks, 1 art, 5 Fighters, 5 Tactical, 4 Bombers.
100% Avg units left for attacker 18 units remaining.
Now lets assume that UK bought 6 inf and a Fighter.
Total Defense: 16 inf, 1 mech, 2 art, 1 tank, 5 AAA, 4 Fighters
Total Attack : 10 Inf, 9 Tanks, 1 art, 5 Fighters, 5 Tactical, 4 Bombers.
90% Avg units left for attacker 10 units remaining.
Now lets assume that UK bought 9 inf.
Total Defense: 19 inf, 1 mech, 2 art, 1 tank, 5 AAA, 3 Fighters
Total Attack : 10 Inf, 9 Tanks, 1 art, 5 Fighters, 5 Tactical, 4 Bombers.
80% Avg units left for attacker 7-9 units remaining.
Looks like 9inf is the safest buy.