That is correct. Vouchers have a 2yr validity period, after which they expire. However, if you had a £10 voucher and you only used/credited £2.50 to BA Miles, then the £7.50 "change" would be recredited to your Tesco Account for issue next quarter.

On your wider point, you will need to phone Tesco to query what figure you are entitled to. Better still explain it on an email and you have their written response as proof. If they say all £123 is yours, then spend it with glee.