Why would it be more avios as two one ways? That makes no sense.

In terms of TFC, and specifically carrier surcharge it is not calculated as the sum of two one way fares.

For MIA-LHR in J per person:
BA YQ surcharge (YQ) $414.00
US International Departure Tax (US) $17.80
US September 11th Security Fee (AY) $5.60
US Passenger Facility Charge (XF) $4.50
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Total TFC is $441.90

For LHR-SFO in J per person:
United Kingdom Air Passenger Duty Apd (GB) $178.10
USDA APHIS Fee (XA) $3.96
US Immigration Fee (XY) $7.00
US Customs Fee (YC) $5.50
United Kingdom Passenger Service Charge (UB) $50.80
BA YQ surcharge (YQ) $176.30
US International Arrival Tax (US) $17.80
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Total TFC is $439.46

For MIA-LHR-SFO in J per person:
BA YQ surcharge (YQ) $828.00
US International Departure Tax (US) $17.80
US September 11th Security Fee (AY) $5.60
US Passenger Facility Charge (XF) $4.50
United Kingdom Air Passenger Duty Apd (GB) $178.10
USDA APHIS Fee (XA) $3.96
US Immigration Fee (XY) $7.00
US Customs Fee (YC) $5.50
United Kingdom Passenger Service Charge (UB) $50.80
US International Arrival Tax (US) $17.80
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Total TFC is $1,119.06

Based on the above your saving booking as two one ways is $237.70 per person. You would lose part of the benefit of the GUF since it would only apply to one of the one way bookings. It would be the same in terms of avios though.