I think the "tall" towel rads are probably at the root of your problem.
There should be a fair distance from the highest point in the circuit to the F&E tank greater than the head of the pump.
This creates a positive head of water sufficient to counteract the suction effect which the pump has on a conciderable lenth of circuit, which would otherwise encourage air infiltration.(as in your situation).
Usually a height of 600-700mm from floor to top of highest rad in circuit for the above conditions to be satisfied.
If the pump has "head" adjustment, this may solve your problem.(lower head setting)
If it hasn't got or doesn't solve, then other than fitting lower towel rads, the pump would have to be fitted in the flow pipe and not in the retun pipe as it probably is presently.
The the tanks would have to be raised in the loft.
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