I would try booking over the phone. I have never managed to get BA.com to price those GUF taxes & fees differentials correctly. Assuming the infant is in-lap, the total differential should be around £255, according to Matrix.
Premium Economy:
Fare for 2 adults
Fare 1: Carrier BA TNN0Y6C0 KUL to MAN (rules)
Passenger type ADT, round trip fare, booking code T, K
Covers KUL-LHR (Premium economy), LHR-MAN (Economy)
£302.99
Fare 2: Carrier BA TNN0Y6C0 MAN to KUL (rules)
Passenger type ADT, round trip fare, booking code K, T
Covers MAN-LHR (Economy), LHR-KUL (Premium economy)
£302.99
United Kingdom Passenger Service Charge Departures (UB)
£59.51
BA YQ surcharge (YQ)
£366.20
Malaysia Passenger Service And Security Charge (MY)
£13.10
Malaysia Goods And Services Tax (D8)
£0.80
United Kingdom Air Passenger Duty APD (GB)
£150.00
Subtotal per passenger
£1,195.61
Number of passengers
x2
Subtotal For 2 adults
£2,391.22
Changes to this ticket will incur a penalty fee.
Fare Construction (can be useful to travel agents)
KUL BA X/LON BA MAN 395.50TNN0Y6C0 BA X/LON BA KUL 395.50TNN0Y6C0 NUC 791.00 END ROE 4.2648 XT 0.80D8 13.10MY 150.00GB 59.51UB 366.20YQ
Fare for 1 infant in lap
Fare 1: Carrier BA TNN0Y6C0 KUL to MAN (rules)
Passenger type INF, round trip fare, booking code T, K
Covers KUL-LHR (Premium economy), LHR-MAN (Economy)
£30.29
Fare 2: Carrier BA TNN0Y6C0 MAN to KUL (rules)
Passenger type INF, round trip fare, booking code K, T
Covers MAN-LHR (Economy), LHR-KUL (Premium economy)
£30.29
United Kingdom Passenger Service Charge Departures (UB)
£46.17
BA YQ surcharge (YQ)
£43.20
Subtotal per passenger
£150.37
Number of passengers
x1
Subtotal For 1 infant in lap
£150.37
Changes to this ticket will incur a penalty fee.
Fare Construction (can be useful to travel agents)
KUL BA X/LON BA MAN 39.55TNN0Y6C0 BA X/LON BA KUL 39.55TNN0Y6C0 NUC 79.10 END ROE 4.2648 XT 46.17UB 43.20YQ
Total Airfare & Taxes
£2,541.59
BUSINESS
Fare for 2 adults
Fare 1: Carrier BA INN7Y6A0 KUL to MAN (rules)
Passenger type ADT, round trip fare, booking code I, J
Covers KUL-LHR (Business), LHR-MAN (Business)
£1,048.48
Fare 2: Carrier BA INN7Y6A0 MAN to KUL (rules)
Passenger type ADT, round trip fare, booking code J, I
Covers MAN-LHR (Business), LHR-KUL (Business)
£1,048.48
United Kingdom Passenger Service Charge Departures (UB)
£69.73
BA YQ surcharge (YQ)
£452.60
Malaysia Passenger Service And Security Charge (MY)
£13.10
Malaysia Goods And Services Tax (D8)
£0.80
United Kingdom Air Passenger Duty APD (GB)
£176.00
Subtotal per passenger
£2,809.23
Number of passengers
x2
Subtotal For 2 adults
£5,618.46
Changes to this ticket will incur a penalty fee.
Fare Construction (can be useful to travel agents)
KUL BA X/LON BA MAN 1368.58INN7Y6A0 BA LON BA KUL 1368.58INN7Y6A0 NUC 2737.16 END ROE 4.2648 XT 0.80D8 13.10MY 176.00GB 69.73UB 452.60YQ
Fare for 1 infant in lap
Fare 1: Carrier BA INN7Y6A0 KUL to MAN (rules)
Passenger type INF, round trip fare, booking code I, J
Covers KUL-LHR (Business), LHR-MAN (Business)
£104.85
Fare 2: Carrier BA INN7Y6A0 MAN to KUL (rules)
Passenger type INF, round trip fare, booking code J, I
Covers MAN-LHR (Business), LHR-KUL (Business)
£104.85
United Kingdom Passenger Service Charge Departures (UB)
£56.39
BA YQ surcharge (YQ)
£43.20
Subtotal per passenger
£309.59
Number of passengers
x1
Subtotal For 1 infant in lap
£309.59
Changes to this ticket will incur a penalty fee.
Fare Construction (can be useful to travel agents)
KUL BA X/LON BA MAN 136.86INN7Y6A0 BA LON BA KUL 136.86INN7Y6A0 NUC 273.72 END ROE 4.2648 XT 56.39UB 43.20YQ
Total Airfare & Taxes
£5,928.05