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If you are going aboard at southampton it is very unlikely that you will have a problem. I also take water on board in 5 litre bottles and have no problem so mixers should not give you a problem either. No comments about the water please from some of the more ascerbic commentators - I have reasons.
Luggage is scanned. We were given some minature bottles of wine on the aeroplane for our Wedding Anniversary. Didn't drink all and put three in the luggage. Didn't get that case until the following day, as it was caught on the scanner and we had to hand the wine in to ship personnel.