I was in MOD 1989 to 1994 in a couple of posts seeing circulation papers when the RAF was offered the F-15E at a preferential price to extend production for a lot less than Harrier GR5 was costing at the time. Given that the main raison d'etre for Harrier GR5 and later models - forward field operations in Germany - had gone by then, there is no doubt in my mind which the better bang for the buck would have been in the long term.
The sheer combination of performance and weapon load carrying ability - both in weight and variety terms - of the F-15E is unsurpassed in any fighter even today.
Whether the RAF would ever have been able to avail itself of all the offered upgrades - both to the aircraft and in terms of weapons options - over the last 25 years is another matter.
However, had the F-15E been procured, both the Jaguar and the Harrier could have been binned earlier than they were, with significant savings in support costs - one fleet instead of two - and there would have been a lot less messing around with fleet-within-fleet issues to get an acceptable theatre entry standard for the deployed operations that followed.
F-15Es bought in the mid-1990s would most probably still be on the RAF's front line now.
Taking things a step further - although it's unlikely to happen in the UK - for any small air arm looking to expand it's capability exponentially in a relatively short timescale - and or to augment its relatively small buy of F-35s - the F-15EX offers a lot of capability for the money.
Even the USAF - with its ongoing ageing fighter fleet recapitalization issue - is just beginning to realize this.
This is an interesting article on the F-15EX debate going on in the USA - with plenty of pictures of F-15E weapon load combinations, details of F-15EX upgrades, and descriptions of various capabilities that have emerged over the years and others that could be developed in the future.
https://warontherocks.com/2019/06/f-...ighter-debate/