It is possible to go SAR straight from Shawbury and some have done so in recent years. However its the numbers game. If the requirement is for SH then they get the lions share, if there are SAR slots available then if you star on the SARTU phase and do well during the whole of the Shawbury course you might get a chance. Otherwise as previously mentioned, you may have to do a couple of tours on SH before you get the chance to crossover. How many slots will be available in the future? anybody's guess.
Best of luck for the future, work hard and dont relax during training.