i can say for sure the few times ive had issues transferring money overseas tesco customer services claim to be checking the status on an hsbc system

Maybe their forex is via Hsbc?

I think you might mean SWIFT (wordlwide) or SEPA (euro area + other EU).

Forex typically means buying or selling currencies, not transferring money between parties.

It's a moot point, but forex's are also used to move money internationally. It is ultimately buying and selling currencies, but you get much better rates than through the bank because the forex isn't actually moving your money. It's receiving money into accounts which are in the senders country, and then, using its accounts in the receivers country, it pays the receiver.

Also means the receiver doesn't get hit with fees to receive an international payment.

Some of the smaller banks and building societies seem to use larger banks like HSBC to process transfers.