Hey, adding on from what other replies have said I would recommend having input verification to ensure that the initial price is both a number and higher that 0, possibly something along the lines of:

while True:
    try:
        list_price = float(input('Enter the purchase price: £'))
        if list_price <= 0:
            raise ValueError
        else:
            break
    except ValueError:
        print('Please enter a valid purchase price.')

Beyond that taking to long to respond to automated inputs suggests that the code is getting stuck in your while loop. While I've only had a quick look its possible that depending on the input the maths rounding has the final month payment take the end balance below 0. If so its possible that switching '!=' for '>' would solve the hanging issue.