Questions about flapping and glottalisation.

Hello. I'm a student from Korea (South) trying to learn English based on received pronunciation. And it's kind of hard because everyone here tries to learn American English.

Here goes the questions:

1. I reckon Americans often use 'flapped d', but it seems that British people don't flap 'd' sound as much as Americans do.
Is it considered 'lazy' to flap 'd' pronunciation in British English? Like, when you say 'had a', or 'bad eighties'.

2. How about flapping 't' sound in one word? i.e., little, bottle...

3. Using glottalisation is THAT bad? I know it's originated from Cockney accent but I love the sound when 't' is glottalised.
If I glottalised the 't' sound when it's in end of the word, for example, 'I got a', 'about it', would I be considered as a 'lazy' speaker?

4. Am I using 'reckon' properly?

Sorry for asking so many questions, but I'm a very curious student.

Thanks for reading. Have a good one!