The consular stage is not in this route
To legalize documents for Portugal you use a single-step certification made in Britain, and that is the end of it. No mission is involved and none can be added. Said plainly at the start, that fact changes what you should be thinking about for the rest of the job.
Count the handovers, not the stamps
A useful way to size any legalisation job is to count how many times the paper leaves your control. Each stage is two journeys, out and back. Embassy attestation for Portugal would add a further pair, and it does not exist here, so the count stops at two. Two is close to the floor for any job of this kind.
That low count is the practical benefit of the short route, and it is a bigger one than the money. Consular legalization for Portugal would not only cost more, it would put a file that cannot be duplicated into the post twice more. Fewer journeys means fewer chances for a sorting office to have a bad week.
Protecting the journeys you do have
Since both handovers are postal, the sensible spending is on the post itself. Track the envelope out and track it home. Put the return label in with the file rather than hoping one is provided. Note the date it left, because a range of days only helps you if you know when the count began.
The other half of the defence is the paper you chose. Southampton Register Office at 6a Bugle Street, Southampton SO14 2AJ prints fresh certified copies of birth, marriage and death entries, so what travels is replaceable. Combine a replaceable copy with a tracked envelope and the worst realistic outcome is a delay, not a disaster.
Cost and calendar
An official fee applies for the British certification, charged for each paper rather than for the envelope, so putting three records through costs three times as much as one. Standard handling runs 7–10 business days and faster handling runs 2–3 business days, and both of those figures already contain the two journeys.
A planning note
Anyone arranging embassy legalization for Portugal from Southampton is really arranging one round trip in the mail. Plan it as you would any other: send early in the week, keep the tracking reference somewhere you will find it, and do not let the file sit on a desk at home for three days before it goes. Document legalization for Portugal is short enough that your own delays become the largest part of it.