UK Polling Report provide two seat estimators that use Universal Swing. As well as a regional predictor that allows you to apply a swing to the different regions and nations of the United Kingdom. As in common with most swing predictors, this excludes Northern Ireland.
Electoral Calculus provide a "strong transitional model" seat estimator that uses a statistical model to predict seats that will switch parties. This model makes some basic assumptions about Northern Ireland seats to include them. An AV simulator is available, but it is unsure on how second/third preferences are allocated and calculated.
Scotland Votes provide a seat calculator for the more complicated Scotish Parliament vote, which takes account of the FPTP constituency seats and the "Top-Up" STV regional seats.