Answer:
A British 1840 silver Groat (Fourpence)(QV), uncirculated and in absolute mint condition could fetch up to £125 GBP. If it has been circulated but still in good condition, it might fetch anything from £5 to £45 GBP.
A British 1840 silver Groat (Fourpence)(QV)(with small round "o"), uncirculated and in absolute mint condition could fetch up to £150 GBP. If it has been circulated but still in good condition, it might fetch anything from £8 to £50 GBP.
A reputable coin dealer will be able to give a more accurate valuation.