rcd protected or not - as long as the rating of the boiler is below 3000w, then its ok on the ring circuit. assuming its a gas boiler, it shouldnt be much more than 300w.
If you are just installing the boiler then for convenience sake it is fine to take a fused spur off of the ring.
If you are doing a complete rewire then I would put it on a B20 MCB protected 2.5mm Radial on the Non-RCD protected side of the DB.
This will slightly lessen the loading of your ring (admittedly, not by much!) but more importantly, any future nuisance tripping of the RCD will not disable the heating.
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