1.These are technically coins, but not "British coins". See the letters "TDC" in the obverse legend? That's short for "Tristan da Cunha", a tiny volcanic island in the South Atlantic, that is often used by these mints as a flag-of-convenience. Technically part of the St Helena & Ascension Islands dependency, it has long issued its own NCLT coins. While nominally legal tender, for 25 pence each, I'm not sure if they are actually redeemable on the island or anywhere else. "One crown" equals 25 British pence, so we're not talking high face value here.
2. I don't know if there are any limits. Bradford doesn't have any minting machinery, so would have had these coins produced on commission, probably made in China or wherever else is cheap right now.
3. Collector interest in mass-marketed NCLT is very minimal. You're more likely getting interest from non-coin-collectors wanting some kind of souvenir of the Queen.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis