Count the sides first
Almost nobody sends one page to Bangladesh. It is a birth entry with a marriage entry, or a degree with a transcript behind it. Because the rate is £25 a page and a page means a printed side, five minutes with the stack tells you the cost before you commit. It also shows up the sheet that is missing, faded or folded — the one that would have held everything else up.
Bengali is what the file must be in
A certified translation into Bengali in London is required for papers that will be used in Bangladesh, whatever language they were issued in. English on its own gives a clerk nothing to act on. The Bengali becomes the text that is read, and your English page rides beside it so a name, a date or a registry number can be traced back to where it came from.
One hand, one spelling
Names are where mixed stacks come apart. A surname can be written more than one way in Bengali script, and two translators working a week apart may not land on the same choice. Sending everything at once keeps the whole set in one hand, so a father's name reads the same on the birth entry as it does on the school record. Consistency across the stack is worth more than an early start on one page.
Who signs, and what they sign
An official Bengali translation in London is finished with a certificate of accuracy: a short statement, dated, naming the paper and the person who wrote the Bengali. A certified Bengali translator puts their name to it, which moves the risk of a wrong word off you and onto them. Notarisation can be added where a file calls for it, but the signed certificate carries the text on its own.
Keeping your own set intact
A certified translation in London for Bangladesh runs entirely from scans, so nothing you own goes into an envelope. Where a record is a civil one, a fresh certified copy from Westminster Register Office can stand in for the sheet you keep, which matters when the same birth entry will be wanted again for a visa or a school place. Order it early and the rest of the stack is not left waiting.