I have never had a package sent from Australia but from America I normally get stuff between 4 and 14 days.
If it is under 18 worth of goods you wont have to pay import tax, if it is over the package could take longer to arrive if customs intercept it (think they are on the ball with this these days). If it is spotted and you are due import tax on it royal mail will charge you the import tax plus a handling fee of 8 or so. Import tax is different on different goods so no real way of telling how much extra you may have to pay.
So depending on the price of that bath mat it could become very costly. But I doubt a vegetarians bathmat is worth more than 18 anyway.
I once had a small tft screen delivered from holland to my work, the guy who took delivery of it for me asked WTF I was getting from Holland, he misheard my reply of a Screen and genuinely thought I had imported 2 kilos of Amsterdam's finest and was at me for some for a couple of days.
FFS I can babble a lot of irrelevant shite sometimes. I blame the screen.