The previous two posts sum up LoCo to BA LH pretty well. Overall it represents a reduction in stress, and an increase in quality of life. Not quite as cushy as it used to be, but little changed frankly.
OTOH, move from LoCo to BA short haul, and you are likely to get rather a shock! You will spend far more time in uniform than you used to. More days at work, and longer days at that! Coupled with min rest night stops, or with more than a 60 minute commute to Heathrow, and you could easily, like I have, look back on the previous 6-7 days, and realise you have worn nothing but uniform! BA will issue you with, I think, 6 shirts. That's not nearly enough. They, and the rest of the uniform will inevitably start to look very shabby, very quickly!
BA weren't interested in an 'Easy' style pattern of work, as they wanted more days at work out of their crews!! They also like their pilots to hang around in a cafe for hours on end, waiting for their next aircraft to arrive. Meaning you'll probably fly fewer hours in your increased days at work!
But, as with many things, there is a silver lining. Anyone joining recently, with sufficient experience for commands, can be the captain of an A319/320/321 within a year. Because so few people who have escaped to LH can stomach the idea of returning!
So basically, if all you want to be is a SH pilot, I would recommend staying at Easy, or aiming to work for them!
LH at BA still (currently) remains a very good, and well paid, option.