The larger pre-1997 50 Pence coins were withdrawn from circulation and demonetised in 1998. So, unless they are part of a Proof or uncirculated mint set or are individual Proof or uncirculated coins and in absolute mint condition, they have little or no value.
A British 1979 cupro-nickel 50 New Pence coin (heptagonal)(Elizabeth II), uncirculated and in absolute mint condition could fetch up to £2 GBP.
A British 1979 cupro-nickel 50 New Pence coin (heptagonal)(Elizabeth II)(Proof FDC), could fetch up to £3 GBP.
A reputable coin dealer will be able to give a more accurate valuation.