I'm not sure much leasing out of slots at Gatwick by the likes of BA, Norwegian or Virgin will really happen (I don't count Vueling using BA slots as leasing since it's all within IAG). If leases are done, I imagine the terms will be extremely generous - e.g. for a nominal £1 - this is very much a buyer's market
When Gatwick was fairly full, obtaining slots required more effort - hence worthwhile for Easyjet to enter some sort of leasing relationship with another airline. If there's a major exodus from Gatwick, it's likely that slots will be much more available. Yes, Virgin / BA / Norwegian can say 'if you don't pay we won't lease the slots to you', but I imagine Wizz may call their bluff knowing these airlines will struggle to fulfil the 80% rule, and that ACL or Govt are unlikely to look well at such a claim. A more aggressive airline seeking slots at Gatwick might even decide to sue ACL if Gatwick slots are left unused while the entrant airline cannot gain the slot via the pool
If we assume slots are readily available, why would a new entrant (e.g. Wizz) want to bind itself to Virgin ? Perhaps easier to keep things simple, and for Wizz to just obtain as many slots as they can from the open pool - and not risk any ongoing return-of-lease obligations in case Virgin decide to return to Gatwick in 3 years time