It's 2 avios per pound, if booked to depart from the UK, and of course there can also be an avios disadvantage if you book into a lower class than would have been available on a flight-only booking.

Also consider whether the cost saving and avios outweigh the loss of any cashback from a site such as TopCashback (though you can claim TCB on a BA Holidays booking too) and the potential loss of on-property benefits such as points earnings, breakfast, or wifii.