What are the dates for your spring break? An initial suggestion would be to fly from LHR or London-Gatwick to Rome (FCO), travel to Florence and then to Venice (VCE). Or, do it in reverse if you prefer. Then, fly to London from Venice. Flying these one-way trips on the inexpensive airlines can be much less than going by train. The problem is, you need to book these flights way in advance to get the cheapest fares. EasyJet has flights from Gatwick to both Rome and Venice. Their fares usually start out at about 30pounds but I'm seeing 66pounds for a flight to Rome on April 6. Again, you should be booking 6 months out to get the cheapest fares.
By train, you are probably looking at close to 200pounds each way and the loss of one full day of travel in each direction.
The next suggestion is to ride the ICPlus or R trains while in Italy. The R train is the local and the least expensive. It would take an hour or so longer to get from Rome to Florence than on the faster Eurostar but the fare is 50% less. The ICPlus train is a bit more expensive than the R train but still, it's much less than the Eurostar.
Finally, there is an inexpensive express train from Rome FCO into the main station and there is an inexpensive bus from Venice to the airport.