Sounds too cheap - you'd need 1620 l of foam, or about 65 kg of liquid at an expansion ratio of 25:1 - would probably cost at least £150 - £250.
You can't mould a blank that size in one go.
First, really thick sections tend to overheat and scorch in the middle - don't build up more than about 4" at a time, and preferably less.
Second, if you make large blocks this way the bubbles tend to be rather large, up to perhaps 1/2 inch across or more for a block that size - depends a bit on the foam, of course.
A better idea might be to stick layers of cheap lightweight expanded polystyrene insulation foam together to make the blank's center, perhaps around a scaffold pole, and turn o a rough shape. This inner core would cost about £100 (6 sheets 100 mm x 1200 mm x 2400 mm @ £16 each).
Then either spray with a thinnish layer of polyurethane foam, or glue sheets of "blue" foam to the outside if you need that accuracy. Polyurethane 2-part foam or foam-in-a-can makes a good glue for this if used _very_ sparingly.
You could also/ instead wrap the inner part of the blank with 2mm flexible polystyrene foam, as used for lining walls and ceilings (£2 a roll in a diy store). Gives a nice finish.
Heck, even a thin layer of papier-mache would do for the outside, and can be sanded to a very smooth finish. 2 coats PVA varnish before glassing. Remove traces with enzyme wallpaper remover.
Sounds overly expensive. What kind of foam is that?
The best foam for your purpose would probably be a medium weight expanded polystyrene packing foam (you don't really need blue foam) - try Eccleston & Hart 0121 683 0300 for a quote of MD15. Won't be cheap, but it shouldn't be that much.
If it's stll too expensive give ma an email I may be able to suggest someone else (I know some other suppliers, but they usually have a minimum order size).
4.5 m x 0.6 m dia of softwood is going to cost a whole lot!You'll have them no matter whether the blank is made of foam or wood - but a lathe for foam will cost a whole lot less than a wood lathe, it doesn't need the rigidity a wood lathe needs.
Are you going to do this indoors? Have you got a 5 m high ceiling???
Apart from being far easier to shape and work with, foam has another advantage - you can apply the fibreglass directly to it, cure, and then dissolve the foam (petrol, or a mixture of cellulose thinners and acetone work well) leaving a hollow shell.
No release coat needed. And you can do undercut shapes, which are impossible to mould with a wood mould.
You can't reuse the mould - but afaics you are only trying to build one?