Heathrow is scheduled to 98% of its capacity, itself about 10% more than is desirable, as such a high scheduling places great service pressure on the system that operates it (NATS). Not much headwind reduces capacity to the extent that Heathrow quickly becomes oversubscribed and robustness of service deteriorates accordingly.
In my view, this degree of scheduling puts inappropriate levels of pressure on the Final director to achieve minimum spacing in all conditions, on a near saturated r/t frequency, with a high proportion of foreign operators, using an airspace system that was not designed for this level of traffic - and which currently does not build in a vertical safeguard when traffic is turning in towards each other form opposite sides. Under such circumstances there is little time or, most importantly, ATCO & r/t capacity, to correct errors of judgement and as a result spacing errors often get left uncorrected. NATS' own wake vortex separation figures conclusively prove this.
This situation is the direct result of the interpretation of the government's policy of "...best use..." to mean maximum use. There is no more room at Heathrow; T5 is principally a service improver, with some additional capacity available if a decision is made to increase runway capacity.
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