Speaking as someone who has lived for years in both cities, London is considerably less safe (and I'm not saying it's dangerous, but Portugal is paradise by comparison) and Lisbon has great weather most of the year.
I can see Gyokeres being sold on the package - despite not being Champion's League this year, they could easily do it next year because of the general standard in the Portuguese league being so lopsided. They've finished Top 3 in 8 of the last 10 years - Top 2 automatically qualify for Champion's League and I think 3rd can still qualify.
Top 2/3 is mainly between Sporting, Benfica & Porto every year. So it's an OK domestic league to hopefully get in good form and then the prospect of regularly playing some of the better teams in Europe. I can see how that might appeal vs a bottom half Premiership club where he may not get regular football, or face the club imploding / being relegated within a year.