It's ashame you don't drive as I guess that would make it slightly easier but it's still definitely doable. I'm not sure whereabouts you are in the UK but there are parkdean holiday parks, haven places and Butlins, and centreparcs etc all around the UK so I'm sure it would be fairly easy to find one within easy reach of a train station.
It would be no more travelling than a plane journey and coach journey if you went abroad.
My parents used to take my brother and I abroad every year from when I was 4 years old. I still remember my best holidays being caravan holidays in the UK! Kids really don't care as long as there's fun things for them to do!
I totally get you wouldn't be comfortable flying so late in your pregnancy, although it's safe I also personally wouldn't want to just because I know how uncomfortable I was in those last weeks and can't think of anything worse than sitting on a plane for hours, then a coach, then having to deal with crazy heat! Lol.
I think you'd find a huge difference in temperature between June and August too. My parents always took us away in either May, June or September and it was lovely. But then when I got with oh we started going away in July and August and it was absolutely scorching. I loved it as I'm a sun worshiper but I would never take my girls away in those months while they're still so young.
My eldest was 14 months old when we first took her to Cyprus, we went in September and it was hideously hot. She would get really restless with the heat and not really know what to do with herself. So much of the holiday was sat inside our air conditioned room trying to keep her cool and comfortable, or in the cool bars etc. I dread to think how hot it would have been if we'd have gone in July or August.
Or you could do some family days out instead, trips to places like Legoland and Peppa pig world, or depending where you are in the UK any of the other attractions there are. West Midlands Safari park, flamingo world (or something like that). I'm sure she'd love lots of days out where it's all about her.
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