Sounds /t/ and /d/ in fast speech

[undeddy]

1. Sound /d/ as far as I know is often omitted when it appears in the middle of consonant clusters. That feature dosen't bring any particular difficulties for me. I also noticed that when I try to say some words containing sound /d/ ('paddle' for instance) fast this sound changes to something like /ɾ/ or /r/. What happens with /d/ sound in UK English (I refer mostly to RP)?

2. The same problems with /t/ sound. Sometimes in my speech it also changes (if I don't try to speak slowly) to something like /ɾ/ or /r/. But British English speakers often change it to the glottal stop /ʔ/ (for example, butter is pronounced like /buʔə/). But for me this is rather difficult as I can't distinguish /ʔ/ and /ə/.
What's the difference between:

/bɒtl/ & /bɒəl/

/lɑːst deɪ/ & /lɑːs deɪ/ & /lɑːsʔ deɪ/

[Tdol]

With your bottle example, it's more like /ʊ/ than a schwa to me- /bɒʔʊl/ rather than /bɒəl/. The glottal stop is not the vowel sound itself, but the break in the flow of air that allows a transition from one vowel to the other.