I am new to travelling to Thailand...so excuse my newb question!

I intend to apply for a 60 day tourist visa into Thailand, then extend it for another 30 days when in Bangkok,

Will i have any issues with airlines or Thai immigration as my return flight will be booked 90 days from departure but i will initially only have the 60 day visa?

Trying to keep the flight cost down so an "open" return flight (if possible) may be more expensive.

What are my best options here for staying 90 days in Thailand?

I live in the UK.

Not 'may be more expensive', will be, a lot. Why don't you ask at your Thai embassy?

Sent from my iPhone using ThaiVisa app

As long as you have a visa airlines and immigration will not have any questions on departure timing.