Can't see why JSF investment would be lost with an F-18 purchase.
JSF would be purchased when it is mature, risk is reduced & is affordable.
F-18 would be passed on to the RAF at a future date which will reduce future spend as a partial GR4 replacement, along with UcAVs and stealthy LRCM, think a pikey FOAS force mix. Look at history with the F4 & Buccaneer being palmed off and put to use elsewhere.
£1.6Bn spent now, will be much cheaper than trying to retrofit in the future when it's realised that cat & traps are required for UCAV operations and the carriers become obsolete half way through their service life, not to mention the risk of 'Dave-B' being canned.
How useful is a 5th generation fighter if it's unaffordable & has maintenance issues ?
It basically fills a capability gap now, and puts off the full JSF spend to hopefully a point in the future when the country can afford it.