How difficult is it to get my Thai girlfriend a visa to visit me in London?

Hello everyone, First sorry about my English it's not my mother language, I hope you guys understand me!

I'm planning to visit thailand This week for visiting my girlfriend there i was with her before! But the first time we traveled together From my country to thailand so we passed the immigration office together!

Now I have An invitation letter from her and i have my tourist visa do it's ok to tell the immigration officer i'm planning for visiting my girlfriend or I'll get a problem because I'm traveling alone this time, Or i can say that I'll visit my friend because i can't find any " visa for visiting girlfriend/ boyfriend "

Please I need help! Any advice

Thank you so much

Ps: as i mentioned I before have already my visa tourist 60 days