Of course it would - that's why it's been their policy for years. At the 2005 election, they won 62 seats. If they had won seats in proportion to the number of people that voted for them, they would have had 161. Not enough to win but enough for them to be a real irritant to the two big parties.
What would be most likely with proportional representation is the same as happens in most of the rest of Europe - a hung Parliament every single time. The parties would have to get used to clubbing together in coalitions in order to form a stable government, just as happens in Germany, Italy, Ireland...