Great Britain is easy enough for the Tour, because of the Channel Tunnel. As long as places in Nord-Pas-de-Calais or whatever the Ch'ti part of the country is now called following the reorganization are willing to host, so you can make the transition back to France make sense, it's not a problem.
Obviously Andorra, Monaco, San Marino, Vatican City are feasible. In theory for the Giro so should Liechtenstein be, but I see little reason for them to host other than at Malbun, which would be a fair way from the border though certainly doable (I had a mooted Giro double mountain stage with one from around Lecco over Splügenpass, Lenzerheidepass and into Liechtenstein to finish on Malbun, then an ensuing stage from Vaduz over Flüelapass, Ofenpass, Umbrailpass and finishing either at Bormio 2000 or Santa Caterina Valfurva). I don't really see the other four microstates as overseas starts at all; Andorra perhaps slightly different but the other three can only be accessed from France (Monaco) or Italy (San Marino/Vatican City) so it's not really an overseas start and requires less logistical hassle than the Sardinian Giro start, Corsican Tour start or the Vuelta starts in the Canaries in the 80s and the mooted Ceuta and Melilla stages. Although the Giro could obviously start in Monaco and that would be a foreign start
Another Giro option would be Albania. There's an improving infrastructure there and their interest in the sport is increasing, plus a sizable minority population in Italy, and with two teams (one men's, one women's) Italian-based but Albanian-registered and a home Tour now set up, it may be feasible in future, with a fast boat transfer to Bari being the most likely route back to the race's home. I think the Giro could also do Malta likewise, although it is probably too small to host three full stages.
For the Vuelta, Guillén's problem is the recognition of Ceuta and Melilla. If he does stages there, Morocco isn't going to want the race because it doesn't recognize the two cities as Spanish. As a result it's more likely he'd have to do something akin to my Race Design Thread start in Melilla using the same circuit in Melilla (the 1997 national championships course) for a circuit race and a TT. Ceuta offers a more difficult and selective stage, but is also much more easy to connect to the mainland and could easily just host one stage before returning to the mainland somewhere like Algeciras or La Línea de la Concepción. There is another option in the Vuelta of course, unlikely as it may sound, and that's Gibraltar. Politics may render it unlikely, but back in 2010 the Ruta del Sol was going to have a 7km ITT at Our Rock, only for the funding from the Llanito companies sponsoring it to fall short after one of the companies pulled out and the stage was replaced by a TT in Málaga instead. Current political climes make it less likely, admittedly, but this may not always be the case - a short TT before the following stage starts back into Andalucía, especially once the new road is completed meaning you no longer have to cross the runway, wouldn't be impossible.
Although I love the race design possibilities of Ceuta and Melilla, I'd prefer a Morocco start with some creative stage possibilities in the Rif. I did a Vuelta Morocco start in the Race Design Thread, with an ITT in Casablanca, then a flat stage from Casablanca to Rabat, and a hilly stage from Ksar-el-Kebir (Alcázarquivír in Spanish) to Tangier before a ferry to Algeciras, so a rest day really wouldn't be necessary there. Stages across the north coast or using Fés as the depart would also be doable. It might also be a way to kill two birds with one stone as well - start in somewhere like Marrakech or Essaouira to finish the Moroccan part of the race somewhere like Agadir and you could then transfer over to the Canary Islands without a rest day, do stages up to and including stage 6 on the islands, then have the rest day on the Friday to travel back to Iberia and then have the rest days either after stage 12 (rest ensuing Friday) or 15 (rest final Monday). Hey presto, Canarias included in La Vuelta, overseas start, no extra rest day needed.