Looking at the F-15EX, it's primary mission (at present, anyway) is to replace the geriatric F-15C's in ANG and possibly active units. In another role, it's a long-range heavy strike fighter than can carry outsize payloads that can't fit inside F-35 internal bays. Presuming, of course, that the conformal fuel tanks are purchased.
The knock against it is that it's a decidedly non-stealthy aircraft. This would become less of an issue with standoff weapons, of course. Or in an Afghanistan type conflict with little or no SAM threat. But those type conflicts are in the past, you know. Not sure how well it would do in the Ukraine - probably not that well in CAS missions.
Probably too expensive for the RAF, and the Typhoon and F-35 seem to have things well in hand.