Do whatever you want.

I have 4 UK TBs and 1 UK DB. I have siginificantly better luck with the TBs, but the DB is fun once in a while. Go ahead. You never know, NF might just add a Tier 3 bomber and make the UK bomber the ultimate bad boy. Then you'd be upset you didn't have more DBs. Chances of that happening? Zero.

But you can dream about though.

Ok basically:
UK TBs del most damage (compared to other nations TBs).
UK DBs deal the least damage (compared to other nations DBs).

Do not mix TBs with DBs

Shortly put... stay with TBs. If you think they suck then wait for them to become AAW immune or learn how to use them.

again, by sheer experience,

4/4 really fails. :(