Alex, have a trawl through the "commercial" part of the forum, there is a lot of information on Coding.
In terms of a charity offering a safety boat in return for "expenses", the rules have been debated here in the past - but you should read the relevant sections in detail, as my interpretation from memory is that you would NOT need to code the boats.
You would only need to MCA code a vessel if it is being used "at Sea" which means outside Categorised Waters. (http://www.mcga.gov.uk/c4mca/msn1776.pdf
). You may still need local authority approval (or other, e.g. PLA?) to use a boat commercially in categorised waters. Its not clear to me that local authority licensing applies unless you are carrying fare paying PASSENGERS. ie. it appears that safety boats in categorised waters may not be regulated at all, even when opperated fully commercially (there are some threads discussing that here); however within the PLA's area of opperation they may have their own rules.
You are right that the MCA do not issue the coding certificates themselves - they simply set the standard and various certifying bodies do the survey etc. This means that some interpret the rules differently from others - and so e.g. you might get away with people on tubes with one surveryor/company and not another. If it does need coded to Cat 4 then you will need a Liferaft. IIRC Cat 6 might have a way of avoiding the liferaft (but a very restriced area of opperation). If you think it needs coded, set yourself aside a day and read http://www.mcga.gov.uk/c4mca/mcga-mn...23704CA95E9786
EDIT: Just to be clear -
The Merchant Shipping (Vessels in Commercial Use for Sport and Pleasure) Regulations refine what is - and what is not - a 'pleasure vessel'. A pleasure vessel does not require coding. The UK's definition of a 'pleasure vessel' is as follows:
"pleasure vessel" means-
(a) any vessel which at the time it is being used is:
(i) (aa) in the case of a vessel wholly owned by an individual or individuals, used only for the sport or pleasure of the owner or the immediate family or friends of the owner; or
(bb) in the case of a vessel owned by a body corporate, used only for sport or pleasure and on which the persons on board are employees or officers of the body corporate, or their immediate family or friends; and
(ii) on a voyage or excursion which is one for which the owner does not receive money for or in connection with operating the vessel or carrying any person, other than as a contribution to the direct expenses of the operation of the vessel incurred during the voyage or excursion; or
(b) any vessel wholly owned by or on behalf of a members' club formed for the purpose of sport or pleasure which, at the time it is being used, is used only for the sport or pleasure of members of that club or their immediate family, and for the use of which any charges levied are paid into club funds and applied for the general use of the club; and
(c) in the case of any vessel referred to in paragraphs (a) or (b) above no other payments are made by or on behalf of users of the vessel, other than by the owner.