IFR plans are not sent to alternates but VFR plans are. If you look in the UK AIP, some airfields point out that filing a FPL on its own does not constitute a request for PPR (eg Farnborough).
We had a situation one sunday whereby an aircraft inbound to Northolt decided it wished to divert to Farnborough (crosswind I think) having (unknown to TAG and ATC) filed Farnborough as alternate, but was refused as it was not an emergency and all sunday slots were allocated; I believe Brize accepted it eventually; don't know why it didn't try Stansted, Luton or Gatwick though.