Can anyone suggest a 'reasonable amount' for a gift for a long-standing client? Thinking of £250 vouchers. Wouldn't want to get it rejected on expenses.
I think is better you check firstly with your company policy and your head of department (VP).
Gifts to clients, even if the person is retiring, could be considered as a form of corruption and there are stricter rules if those "clients" are government officials or HCPs.
My last personal thought, which can be nothing related to your company policy, 250 GBP seems a very high amount, and never seen in 5 different Fortune100 companies where i did work for.