Primarily, goods on display in a shop window are traditionally considered to be mere "invitations to treat", or otherwise, in terms of seriousness, somewhat below a full offer (Pharmaceutical Society of GB v Boots, Fisher v Bell).
Should a conflict subsequently emerge from this situation, the store may well have a defense in that there appears to be little to indicate that their signage indicates a full offer exists at this stage.
Nevertheless, the statement "Absolutely first come, first served" may instead be dragged in as a rebuttal (with reference to the famous "Carlill v Carbolic Smoke Ball"). Indeed the information given (price, sale, "absolutely" is hard to justify as mere "puff"). Should a court be persuaded that the claims and wording are sufficiently strong so that a reasonable person might consider that they WOULD indeed receive a TV upon payment of 50p, then a full legal offer may be considered to have been made by the advert.
In this case, it appears as if the fellow waiting outside might well attempt to make that case, and cite his own actions in waiting, having cash with him, and so on as evidence, in the event of a dispute.
Did the manager indicate that the deal would have a certain longevity, or that it would only be valid for a certain period (from Monday 9:00 am, etc.), and so on ? Surely he might counter with the defense that in actuality the terms are lacking in specifics in this case. If not so, and then we enter into the question of clarity of terms, but that is a different matter entirely.
Daulia v Four Millbank Nominees, and Errington v Errington & Woods provide authority regarding the binding of the offeror once the offeree has started performance. Of course, a defense here would be straightforward: unlike in Errington (where a significant history of payments were proven) no payments were accepted of William, and unlike Daulia (where terms had already been negotiated, and a verbal unilateral offer made) the offer/ITT as it stands seems somewhat thin. Indeed, both cases already pre-suppose that an offer has definitely been made, which as discussed is far from clear in this case that we have her.
In any case, the signage was taken down prior to William actually entering the store, paying money or any other such action that was actually. William, in this case before us now, would then have to prove that actually waiting outside a shop was somehow an implicit (presumably) part of any offer.
Furthermore in any defense, it is possible that the manager might also wish to call upon the authority of Harris v Nickerson, where a plaintiff attempted to pursure a claim of breach of contract after travelling a great distance in order to attend an auction (cancelled at the last minute). In that case, auction catalogues where deemed indeed to be mere ITT, and the fact that the plaintiff was put to inconvenience was considered immaterial to the matter.
I might imagine that William would have to overcome that paritcular objection in any pursuit of action. After all, nobody TOLD him to camp outside the shop.