There are quite a few UK airlines that require you to have a UK issued licence. I know that some airlines have this policy because of having previous problems with other CAA“s (Italy to mention one) that are "JAA" countries.
The problem is that when people had their LPC/OPC and the TRI/TRE signed the form, the foreign CAA (where the licence was issued) refused to accept the TRI/TRE because he/she was not approved by the local CAA. So, instead of getting in to this problem every now and then they require you to have a UK issued licence.
I had no problem converting my licence...but it was £194!!!!
/CP